Make Period - Period return DateOffset instead of int
Discussed briefly in #20049.

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21314?src=pr&el=h1) Report
> Merging [#21314](https://codecov.io/gh/pandas-dev/pandas/pull/21314?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/45e55af9cf74aede00a8b10f8922537f285a573a?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `71.42%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21314/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21314?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21314      +/-   ##
==========================================
- Coverage    91.9%   91.89%   -0.01%     
==========================================
  Files         154      154              
  Lines       49558    49562       +4     
==========================================
+ Hits        45545    45547       +2     
- Misses       4013     4015       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.27% <71.42%> (-0.01%)` | :arrow_down: |
| #single | `42.02% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21314?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21314/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.67% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21314/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `96.55% <50%> (-0.34%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21314?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/21314?src=pr&el=footer). Last update [45e55af...efd7c75](https://codecov.io/gh/pandas-dev/pandas/pull/21314?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 gentle ping i rebased. ping on green. (note periodically you can rebase long running PR's) Ping thanks!