Fix tzawareness_compat for DatetimeIndex comparisons with NaT
ATM `pd.date_range('2016-01-01', periods=1, tz='US/Pacific') < pd.NaT` raises `TypeError` because `NaT` is tz-naive.  This fixes that.

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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19276?src=pr&el=h1) Report
> Merging [#19276](https://codecov.io/gh/pandas-dev/pandas/pull/19276?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/04a6815f7da0f6d879725d553c7a45f47bb4a15c?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19276/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19276?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19276      +/-   ##
==========================================
- Coverage   91.56%   91.53%   -0.03%     
==========================================
  Files         148      148              
  Lines       48874    48876       +2     
==========================================
- Hits        44751    44741      -10     
- Misses       4123     4135      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.91% <100%> (-0.03%)` | :arrow_down: |
| #single | `41.69% <66.66%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19276?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/19276/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.24% <100%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19276/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19276?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/19276?src=pr&el=footer). Last update [04a6815...c97e166](https://codecov.io/gh/pandas-dev/pandas/pull/19276?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
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