BUG: Preserve timezone in unaligned assignments
- [x] closes #12981 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

A fix for #12981.  When doing a slice assign to a DataFrame where the
right-hand-side was timezone-aware datetime Series which required
realignment to perform the assignment, the timezone would be stripped
from the RHS on assignment.

minor changes. ping when green.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/12982?src=pr) is **83.85%**

> Merging [#12982](https://codecov.io/gh/pydata/pandas/pull/12982?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **+<.01%**

``` diff
@@             master     #12982   diff @@
==========================================
  Files           136        136          
  Lines         49711      49711          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          41679      41683     +4   
+ Misses         8032       8028     -4   
  Partials          0          0          
```
1. File `...as/core/internals.py` (not in diff) was modified. [more](https://codecov.io/gh/pydata/pandas/commit/94451553513be70af21eb8649a9217e10dac27fd/changes?src=pr#70616E6461732F636F72652F696E7465726E616C732E7079) 
   - Misses `-4` 
   - Partials `0` 
   - Hits `+4`

[![Sunburst](https://codecov.io/gh/pydata/pandas/pull/12982/graphs/sunburst.svg?size=660&src=pr)](https://codecov.io/gh/pydata/pandas/pull/12982?src=pr)

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by 9445155
 thanks!
