pythonize cython code
Use python-style type annotations instead of cython-style in a few places.

Use python-style isinstance checks in cases where cython will automatically optimize them into C calls.

Part of the hope is that we can get the code close enough to valid python that we can trick flake8 into working on it.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22638?src=pr&el=h1) Report
> Merging [#22638](https://codecov.io/gh/pandas-dev/pandas/pull/22638?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0976e1261781153603064eb67d66630a099edcd7?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/22638/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22638?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22638   +/-   ##
=======================================
  Coverage   92.17%   92.17%           
=======================================
  Files         169      169           
  Lines       50708    50708           
=======================================
  Hits        46740    46740           
  Misses       3968     3968
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.58% <ø> (ø)` | :arrow_up: |
| #single | `42.35% <ø> (ø)` | :arrow_up: |


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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22638?src=pr&el=footer). Last update [0976e12...f9c0c26](https://codecov.io/gh/pandas-dev/pandas/pull/22638?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks. maybe add a note somewhere that we *can* use type annotation in cython files (*maybe* at the top of each file?) > Use python-style type annotations instead of cython-style in a few places.

Sorry, why do we think this is an improvement? Now we have two ways of doing things? 
Can you provide a bit more background / reasoning? Some discussion in #22180.

Main motivation: I _think_ that if we can get the code close enough to valid python, we can trick flake8 into running on it.