CLN: cleanup libs cimports, remove is_timestamp
- cleanup imports in `_libs.parsers`, including some unused
- a couple of functions from util are used exactly once and only in parsers.  Get those directly from numpy_helper instead of from util
- remove unused `util.is_contiguous`
- remove a bunch of distutils shebang-like declarations that are unnecessary and are a pain for when we _do_ want to run tests with coverage enabled.
- remove `tslib.is_timestamp` (will be helpful for eventual goal of "fixing" Timestamp classmethods)
- remove some leftover constants from tslib, update imports appropriately
rebase and something failing Huh the appveyor error is at build-time complaining "unresolved external symbol strcasecmp".  But that particular cimport isn't changed. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18663?src=pr&el=h1) Report
> Merging [#18663](https://codecov.io/gh/pandas-dev/pandas/pull/18663?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9629fef912dbeca5154e9cf58970bad3e02657f4?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18663/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18663?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18663      +/-   ##
==========================================
- Coverage   91.59%   91.55%   -0.04%     
==========================================
  Files         153      153              
  Lines       51257    51254       -3     
==========================================
- Hits        46949    46927      -22     
- Misses       4308     4327      +19
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.42% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.68% <33.33%> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18663?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/18663/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `94.02% <100%> (-0.08%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18663/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18663/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `64.78% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18663/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18663/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `82.01% <0%> (+0.19%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18663?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/18663?src=pr&el=footer). Last update [9629fef...274dafb](https://codecov.io/gh/pandas-dev/pandas/pull/18663?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!