Cut/paste (most) remaining tz funcs to tslibs/timezones
We're very nearly done with `tslibs.timezones`.  I had hoped to bring over the remaining functions in smaller chunks, but this turns out to be the smallest independent subset that contains `_get_dst_info` and `maybe_get_tz`.

Getting `_get_dst_info` and `maybe_get_tz` separated is a milestone because it allows us to move `tseries.frequencies.Resolution` and related functions into cython without having a dependency on `tslib`.  This in turn gets the dependency on `khash` out of `tslib`.  A few more nice things become feasible.

This is _almost_ pure cut/paste.  The only change I made was replacing `isinstance(tz, string_types)` with `is_string_object(tz)`.  If requested, I'll do a follow-up to de-privatize the names.

`timezones` is within spitting distance of being valid python.  Getting it over that hump would allow linting and coverage that is tough as it is.

Note that if/when #17363 is merged, a bunch of residual imports in `tslib` can be cleaned up.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

```
asv continuous -f 1.1 -E virtualenv master HEAD -b tz -b timezone
[...]
       before           after         ratio
     [fa557f73]       [30c3fa6b]
-          1.58ms           1.07ms     0.68  timeseries.DatetimeIndex.time_reset_index_tz
SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```
 can you run benches for ``-b timeseries`` ```
asv continuous -f 1.1 -E virtualenv master HEAD -b timeseries
          before           after         ratio
     [fa557f73]       [109d605e]
+         234±3μs         527±70μs     2.25  timeseries.SemiMonthOffset.time_begin_decr
+        789±10μs      1.33±0.08ms     1.69  timeseries.Offsets.time_custom_bmonthend_incr_n
+         241±3μs         405±30μs     1.68  timeseries.SemiMonthOffset.time_begin_decr_n
+           348μs            509μs     1.46  timeseries.DatetimeIndex.time_unique
+      6.80±0.2ms       9.74±0.5ms     1.43  timeseries.ResampleDataFrame.time_mean_string
+     6.99±0.07ms       9.94±0.8ms     1.42  timeseries.ResampleDataFrame.time_min_numpy
+          13.0ms       18.3±0.4ms     1.41  timeseries.ToDatetime.time_iso8601_nosep
+           2.13s            2.87s     1.35  timeseries.SemiMonthOffset.time_begin_apply_index
+      25.0±0.7ms       33.2±0.9ms     1.33  timeseries.TimeSeries.time_add_irregular
+      7.24±0.2ms       9.60±0.5ms     1.33  timeseries.ResampleDataFrame.time_min_string
+           1.75s            2.22s     1.27  timeseries.SemiMonthOffset.time_end_incr_rng
+     6.70±0.04ms       8.44±0.2ms     1.26  timeseries.TimeSeries.time_large_lookup_value
+         250±5μs          313±7μs     1.25  timeseries.TimeSeries.time_sort_index_monotonic
+      15.2±0.1ms           19.0ms     1.25  timeseries.ToDatetime.time_format_YYYYMMDD
+           1.78s            2.19s     1.23  timeseries.SemiMonthOffset.time_end_apply_index
+      37.3±0.9ms         45.7±2ms     1.23  timeseries.Iteration.time_iter_datetimeindex_preexit
+         149±4μs          182±3μs     1.22  timeseries.Offsets.time_timeseries_day_incr
+      7.61±0.2ms         9.21±2ms     1.21  timeseries.ResampleDataFrame.time_mean_numpy
+         143±4μs          173±4μs     1.21  timeseries.Offsets.time_timeseries_day_apply
+      17.7±0.5ms       21.0±0.1ms     1.19  timeseries.TimeDatetimeConverter.time_convert
+      84.7±0.6μs          100±4μs     1.18  timeseries.Offsets.time_custom_bday_apply
+           892ms            1.05s     1.18  timeseries.ToDatetime.time_iso8601_tz_spaceformat
+      14.9±0.1ms         17.5±1ms     1.17  timeseries.ResampleSeries.time_timestamp_downsample_mean
+           2.01s            2.32s     1.15  timeseries.SemiMonthOffset.time_begin_decr_rng
+       234±0.9μs         266±10μs     1.13  timeseries.SemiMonthOffset.time_end_decr
+        96.6±3μs          109±3μs     1.13  timeseries.Offsets.time_custom_bday_apply_dt64
+        48.0±1μs         54.0±2μs     1.12  timeseries.Offsets.time_timeseries_year_apply
+           3.72s            4.17s     1.12  timeseries.SeriesArithmetic.time_add_offset_slow
+      19.9±0.5ms       22.0±0.9ms     1.11  timeseries.TimeSeries.time_sort_index_non_monotonic
+           1.64s            1.81s     1.10  timeseries.SeriesArithmetic.time_add_offset_fast
+          1.61ms           1.78ms     1.10  timeseries.DatetimeIndex.time_reset_index_tz
-           2.20s            1.98s     0.90  timeseries.SemiMonthOffset.time_begin_incr_rng
-      9.25±0.2ms       8.11±0.1ms     0.88  timeseries.ResampleDataFrame.time_max_string
-          56.4ms           49.4ms     0.88  timeseries.DatetimeIndex.time_infer_freq_none
-          15.5ms       13.6±0.4ms     0.87  timeseries.ToDatetime.time_iso8601_format
-          9.38ms           8.14ms     0.87  timeseries.DatetimeIndex.time_normalize
-          8.87ms           7.55ms     0.85  timeseries.DatetimeIndex.time_infer_dst
-      34.1±0.9ms       28.1±0.3ms     0.82  timeseries.ResampleSeries.time_period_downsample_mean
-        305±10μs          244±1μs     0.80  timeseries.TimeSeries.time_timeseries_slice_minutely
-          18.2ms           13.7ms     0.76  timeseries.DatetimeIndex.time_dti_factorize
-          22.9ms           16.4ms     0.72  timeseries.DatetimeIndex.time_dti_tz_factorize
-          1.82ms           1.30ms     0.71  timeseries.DatetimeIndex.time_reset_index
-          5.19ms           2.97ms     0.57  timeseries.DatetimeIndex.time_add_offset_delta
-          23.3ms           12.5ms     0.54  timeseries.DatetimeIndex.time_add_offset_fast
-        38.1±1ms       18.4±0.7ms     0.48  timeseries.AsOfDataFrame.time_asof_nan_single
-          6.16ms           2.96ms     0.48  timeseries.DatetimeIndex.time_add_timedelta
-      19.1±0.2ms       8.98±0.3ms     0.47  timeseries.DatetimeAccessor.time_dt_accessor_normalize
-           2.54s            1.19s     0.47  timeseries.DatetimeIndex.time_add_offset_slow
-      23.1±0.8ms       10.8±0.2ms     0.47  timeseries.AsOf.time_asof_nan
-        36.4±2ms       16.9±0.4ms     0.46  timeseries.AsOfDataFrame.time_asof_single
-      12.5±0.8ms       5.72±0.1ms     0.46  timeseries.AsOf.time_asof_nan_single
-         139±4μs         63.2±2μs     0.46  timeseries.AsOf.time_asof_single
-      25.2±0.9ms       11.0±0.3ms     0.44  timeseries.AsOf.time_asof
-        143±10ms        61.7±10ms     0.43  timeseries.AsOfDataFrame.time_asof_nan
-        128±20ms        48.7±10ms     0.38  timeseries.AsOfDataFrame.time_asof
-        149±10μs         50.9±1μs     0.34  timeseries.AsOf.time_asof_single_early
-        547±50μs          166±7μs     0.30  timeseries.AsOfDataFrame.time_asof_single_early
-         153±4μs         44.4±1μs     0.29  timeseries.DatetimeAccessor.time_dt_accessor
-      14.0±0.5ms      2.62±0.09ms     0.19  frame_methods.frame_assign_timeseries_index.time_frame_assign_timeseries_index

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
``` those are wildly inconsitent. do are using affinity? Did the previous asv run on the laptop.  New one on the desktop:

```
asv continuous -f 1.1 -E virtualenv master HEAD -b timeseries
[...]
     [fa557f73]       [109d605e]
+         184±1ms          204±5ms     1.11  timeseries.SeriesArithmetic.time_add_offset_delta
-        202±10ms        182±0.8ms     0.90  timeseries.SeriesArithmetic.time_add_offset_fast
-         653±5ms          586±1ms     0.90  timeseries.SeriesArithmetic.time_add_offset_slow
-     9.57±0.04μs      7.68±0.04μs     0.80  timeseries.Offsets.time_timeseries_year_apply
``` ok that looks good. something failing.ping when green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=h1) Report
> Merging [#17526](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fa557f7391589f351b1260f46b3b3db22492f50b?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17526/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17526      +/-   ##
==========================================
+ Coverage   91.18%    91.2%   +0.02%     
==========================================
  Files         163      163              
  Lines       49545    49606      +61     
==========================================
+ Hits        45179    45245      +66     
+ Misses       4366     4361       -5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <ø> (+0.04%)` | :arrow_up: |
| #single | `40.19% <ø> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `98.26% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.53% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `98.55% <0%> (ø)` | :arrow_up: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.18% <0%> (+0.03%)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.77% <0%> (+0.03%)` | :arrow_up: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `95.57% <0%> (+0.04%)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.98% <0%> (+0.32%)` | :arrow_up: |
| ... and [4 more](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=footer). Last update [fa557f7...0147c24](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=h1) Report
> Merging [#17526](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fa557f7391589f351b1260f46b3b3db22492f50b?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17526/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17526      +/-   ##
==========================================
+ Coverage   91.18%    91.2%   +0.02%     
==========================================
  Files         163      163              
  Lines       49545    49606      +61     
==========================================
+ Hits        45179    45245      +66     
+ Misses       4366     4361       -5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <ø> (+0.04%)` | :arrow_up: |
| #single | `40.19% <ø> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `98.26% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.53% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `98.55% <0%> (ø)` | :arrow_up: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.18% <0%> (+0.03%)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.77% <0%> (+0.03%)` | :arrow_up: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `95.57% <0%> (+0.04%)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.98% <0%> (+0.32%)` | :arrow_up: |
| ... and [4 more](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=footer). Last update [fa557f7...0147c24](https://codecov.io/gh/pandas-dev/pandas/pull/17526?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 ping. thanks; would take a followup for _ renaming as well.