BUG: Concatentation of TZ-aware dataframes 
closes #12396
closes #18447

supersedes #19327 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21014?src=pr&el=h1) Report
> Merging [#21014](https://codecov.io/gh/pandas-dev/pandas/pull/21014?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3d03fdb24b1a9c1438fe65f5242cbea5418787bf?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `96.66%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21014/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21014?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21014      +/-   ##
==========================================
- Coverage   91.82%   91.82%   -0.01%     
==========================================
  Files         153      153              
  Lines       49505    49500       -5     
==========================================
- Hits        45460    45455       -5     
  Misses       4045     4045
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.22% <96.66%> (-0.01%)` | :arrow_down: |
| #single | `41.88% <26.66%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21014?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/21014/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `95.59% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/21014/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.64% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/21014/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `99.17% <96.42%> (-0.01%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21014?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/21014?src=pr&el=footer). Last update [3d03fdb...fdc744a](https://codecov.io/gh/pandas-dev/pandas/pull/21014?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 closed via https://github.com/pandas-dev/pandas/commit/1dcddba2200b89cffe97ae7a32a34cdec3a7c8fb thanks @paul-mannino 