BUG: timezone comparisions are inconsistent, manifesting in bugs in .concat
closes #18523 
superseded #18596
cc @jschendel 
cc @jbrockmendel 
cc @MridulS 
cc @pganssle 
cc @rockg  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19281?src=pr&el=h1) Report
> Merging [#19281](https://codecov.io/gh/pandas-dev/pandas/pull/19281?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/65d1b62dfe46cd5cfd80d8b1aafd633575c72ee6?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19281/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19281?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19281   +/-   ##
=======================================
  Coverage   91.56%   91.56%           
=======================================
  Files         148      148           
  Lines       48874    48874           
=======================================
  Hits        44751    44751           
  Misses       4123     4123
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.93% <100%> (ø)` | :arrow_up: |
| #single | `41.69% <66.66%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19281?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/19281/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.23% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19281?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/19281?src=pr&el=footer). Last update [65d1b62...e95f16f](https://codecov.io/gh/pandas-dev/pandas/pull/19281?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
