BUG: Preserve column metadata with DataFrame.astype
- [X] closes #19920
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Follow-up to #18099
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19948?src=pr&el=h1) Report
> Merging [#19948](https://codecov.io/gh/pandas-dev/pandas/pull/19948?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c8859b57b891701f250fb05f2cc60d2e6cae2d6b?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19948/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19948?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19948      +/-   ##
==========================================
+ Coverage   91.66%   91.66%   +<.01%     
==========================================
  Files         150      150              
  Lines       49027    49028       +1     
==========================================
+ Hits        44940    44941       +1     
  Misses       4087     4087
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.04% <100%> (ø)` | :arrow_up: |
| #single | `41.83% <40%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19948?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19948/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.89% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19948?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/19948?src=pr&el=footer). Last update [c8859b5...20c1bd8](https://codecov.io/gh/pandas-dev/pandas/pull/19948?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!