BUG: Handle Period in combine
Followup to #23079 to unxfail a period test.
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/dtypes/cast.py`](https://github.com/TomAugspurger/pandas/blob/5e663c873575757b6a0d85a7e47b350367b8f664/pandas/core/dtypes/cast.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_combine_concat.py`](https://github.com/TomAugspurger/pandas/blob/5e663c873575757b6a0d85a7e47b350367b8f664/pandas/tests/frame/test_combine_concat.py) !

 lgtm. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23350?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@df4ffc7`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23350/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23350?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #23350   +/-   ##
=========================================
  Coverage          ?   92.22%           
=========================================
  Files             ?      169           
  Lines             ?    51261           
  Branches          ?        0           
=========================================
  Hits              ?    47277           
  Misses            ?     3984           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.66% <100%> (?)` | |
| #single | `42.23% <50%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23350?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/23350/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `89.28% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23350?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/23350?src=pr&el=footer). Last update [df4ffc7...5e663c8](https://codecov.io/gh/pandas-dev/pandas/pull/23350?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
