remove period_helper from non-period reqs
There are a bunch of other reqs that _could_ be removed (e.g. every direct reference to src/util, all of `lib_depends`), but at least `period_helper` it's obvious _why_ it belongs where it does.

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

Hello @jbrockmendel! Thanks for submitting the PR.

 - In the file [`setup.py`](https://github.com/pandas-dev/pandas/blob/92251f445cb31f9cfa2b9cdc50270bfea238a582/setup.py), following are the PEP8 issues :

> [Line 480:80](https://github.com/pandas-dev/pandas/blob/92251f445cb31f9cfa2b9cdc50270bfea238a582/setup.py#L480): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (84 > 79 characters)

 looks fine. ping on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17531?src=pr&el=h1) Report
> Merging [#17531](https://codecov.io/gh/pandas-dev/pandas/pull/17531?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/97abd2c9c11aeee0e3d2c58a74d85fa75062ca1f?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17531/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17531?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17531      +/-   ##
==========================================
- Coverage   91.24%    91.2%   -0.05%     
==========================================
  Files         163      163              
  Lines       49582    49586       +4     
==========================================
- Hits        45242    45225      -17     
- Misses       4340     4361      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.2% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17531?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17531?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17531?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17531?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17531?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `98.26% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/17531?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `95.57% <0%> (+0.04%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17531?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/17531?src=pr&el=footer). Last update [97abd2c...92251f4](https://codecov.io/gh/pandas-dev/pandas/pull/17531?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 ping. thanks!