BUG: bug in .at/.loc indexing with a tz-aware columns
closes #15822
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15827?src=pr&el=h1) Report
> Merging [#15827](https://codecov.io/gh/pandas-dev/pandas/pull/15827?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1dab800b412be3613e8f666eb1be88458b631312?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15827/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/15827?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15827      +/-   ##
==========================================
- Coverage   90.99%   90.98%   -0.02%     
==========================================
  Files         143      143              
  Lines       49403    49408       +5     
==========================================
- Hits        44956    44953       -3     
- Misses       4447     4455       +8
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15827?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/15827?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.56% <100%> (-0.1%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/15827?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15827?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.3% <0%> (+0.33%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15827?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/15827?src=pr&el=footer). Last update [1dab800...4fcd2c6](https://codecov.io/gh/pandas-dev/pandas/pull/15827?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).