COMPAT: NaT support tz_localize / tz_convert (#15830)
closes #15830 

* add tz_convert/tz_localize methods
* add tests

 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15868?src=pr&el=h1) Report
> Merging [#15868](https://codecov.io/gh/pandas-dev/pandas/pull/15868?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/74f527ff0cbc8045b9f350382a4ad37694e8c5e6?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15868/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/15868?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15868      +/-   ##
==========================================
- Coverage   90.98%   90.96%   -0.02%     
==========================================
  Files         143      143              
  Lines       49449    49449              
==========================================
- Hits        44992    44983       -9     
- Misses       4457     4466       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.72% <ø> (ø)` | :arrow_up: |
| #single | `40.65% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15868?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/15868?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/15868?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.56% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15868?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/15868?src=pr&el=footer). Last update [74f527f...7c8ae63](https://codecov.io/gh/pandas-dev/pandas/pull/15868?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). can you add a whatsnew note as well (add this issue number next to #15782 in the whatsnew) Thanks, I erased the unnecessary methods and add the wathsnew note   thanks @atbd nice PR!