implement constructors for TimedeltaArray, DatetimeArray
Implements AttributesMixin since this is a pattern we might want to formalize.

The `_simple_new` that this implements for TimedeltaArray and DatetimeArray are pretty close to the "full" versions in the Index classes.  The `__new__` methods though are very much stripped down for the now.

With these in place we get to move `tz_convert` and `tz_localize`.  Could do a bunch more if I wasn't too much of a wuss to do it all in one go.
Just updated to de-conflict changes to tz_convert/tz_localize docstrings. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21803?src=pr&el=h1) Report
> Merging [#21803](https://codecov.io/gh/pandas-dev/pandas/pull/21803?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5cb5880d8f1647e303ba52487dc867db838fd6e0?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `79.1%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21803/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21803?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21803      +/-   ##
==========================================
- Coverage   91.95%    91.9%   -0.05%     
==========================================
  Files         160      160              
  Lines       49858    49902      +44     
==========================================
+ Hits        45845    45864      +19     
- Misses       4013     4038      +25
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <79.1%> (-0.05%)` | :arrow_down: |
| #single | `42.1% <64.17%> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21803?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21803/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `94.4% <ø> (-0.04%)` | :arrow_down: |
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21803/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `85.71% <100%> (-0.65%)` | :arrow_down: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21803/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.01% <100%> (+0.15%)` | :arrow_up: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/21803/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.06% <100%> (+0.05%)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21803/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.13% <100%> (-0.1%)` | :arrow_down: |
| [pandas/core/arrays/timedelta.py](https://codecov.io/gh/pandas-dev/pandas/pull/21803/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YS5weQ==) | `87.65% <64%> (-10.63%)` | :arrow_down: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21803/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `93.62% <70.45%> (-6.38%)` | :arrow_down: |
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21803/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `93.71% <90%> (-1.98%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21803?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/21803?src=pr&el=footer). Last update [5cb5880...192c920](https://codecov.io/gh/pandas-dev/pandas/pull/21803?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel 