Finish moving period_helper to cython
- [x] closes #19465

There is probably some cleanup that can be done now that period logic is self-contained.

No evident perf impact.
```
asv continuous -E virtualenv -f 1.1 master HEAD -b period
[...]
       before           after         ratio
     [776fed3a]       [7cc0d9e8]
+        1.14±0μs      1.28±0.01μs     1.13  period.PeriodProperties.time_property('M', 'qyear')
-        579±30ms          509±4ms     0.88  groupby.Datelike.time_sum('period_range')
```

(will post results from a couple more runs as they become available)
<b> update<b/> two more runs show zero change, not even noise.

<b>update</b> Oh, also did some unrelated modernizing of string formatting in timedeltas.
https://travis-ci.org/pandas-dev/pandas/jobs/412170226#L3465

Looks like you have some linting failures. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22196?src=pr&el=h1) Report
> Merging [#22196](https://codecov.io/gh/pandas-dev/pandas/pull/22196?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/776fed3ab63d74ddef6e5af1a702b10c2a30bbb6?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22196/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22196?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22196   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         169      169           
  Lines       50694    50694           
=======================================
  Hits        46671    46671           
  Misses       4023     4023
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <ø> (ø)` | :arrow_up: |
| #single | `42.32% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22196?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/22196?src=pr&el=footer). Last update [776fed3...48e273b](https://codecov.io/gh/pandas-dev/pandas/pull/22196?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 https://travis-ci.org/pandas-dev/pandas/jobs/412207881#L3475

Looks like you (still) have some linting failures. 🙁  https://travis-ci.org/pandas-dev/pandas/jobs/412219771#L3474

Getting there, but not quite.  Are you able to check locally via [cpplint](https://github.com/pandas-dev/pandas/blob/2625759abeb78655558067d55a23c293628c3165/doc/source/contributing.rst#c-cpplint)? > asv continuous -E virtualenv -f 1.1 master HEAD -b period

can you create an issue to add more period benchmarks
 > can you create an issue to add more period benchmarks

Done thanks!