Series.describe returns first and last for tz-aware datetimes
GH issue 21328

- [x] closes #21328 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21332?src=pr&el=h1) Report
> Merging [#21332](https://codecov.io/gh/pandas-dev/pandas/pull/21332?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d24a9507ba539f455d9b90885edf098c7dc93e99?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21332/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21332?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21332      +/-   ##
==========================================
+ Coverage   91.93%   91.94%   +<.01%     
==========================================
  Files         159      160       +1     
  Lines       49733    49738       +5     
==========================================
+ Hits        45721    45730       +9     
+ Misses       4012     4008       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.31% <100%> (ø)` | :arrow_up: |
| #single | `41.99% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21332?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/21332/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.45% <100%> (+0.24%)` | :arrow_up: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/21332/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.93% <0%> (-0.09%)` | :arrow_down: |
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/21332/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `92.65% <0%> (-0.02%)` | :arrow_down: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/21332/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `97.01% <0%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21332/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.33% <0%> (ø)` | :arrow_up: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/21332/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `95.45% <0%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21332?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/21332?src=pr&el=footer). Last update [d24a950...0af4758](https://codecov.io/gh/pandas-dev/pandas/pull/21332?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 It would be good to add (or see if there's) a test for `DataFrame.describe()` with a datetime tz dtype column as well.  I looked through the existing tests for `DataFrame.describe()` (in `frame/test_analytics.py`). It looks like they are all testing logic about `include`/`exclude`, or what happens with column names - not the core logic of what happens when each Series is passed to `describe_1d`. So I don't think it makes sense to add a new test there for this functionality, which is all within the `describe_1d` call and therefore tested by the tests on `Series`. @jreback made the suggested change, ready to merge or further changes desired? Changes made:
* add to whatsnew
* use `tz_naive_fixture`
* change name of series test

Ready for re-review Just a few documentation comments, one merge conflict, otherwise LGTM. Thanks @mroeschke  . fixed. Thanks @louispotok! 