STY: standardize spacing for casting, with linting
Casting in cython is done with the syntax `<type>obj` or `<type> obj`.  ATM the code uses both of these.  This PR standardizes on the first usage, with a lint check.

@datapythonista I'm not sure whether the grep check here belongs in the patterns section or in the non-python section.  LMK if you have a preference.
> prob should put all of these rules into a flake-cython package :)

If we can find someone else to maintain it, that'd be fantastic. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23474?src=pr&el=h1) Report
> Merging [#23474](https://codecov.io/gh/pandas-dev/pandas/pull/23474?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d78bd7a65f87022bae5021d36c8656c0ea5a7b76?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/23474/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23474?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23474   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files         161      161           
  Lines       51197    51197           
=======================================
  Hits        47220    47220           
  Misses       3977     3977
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.27% <ø> (ø)` | :arrow_up: |


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[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23474?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23474?src=pr&el=footer). Last update [d78bd7a...8d5e69c](https://codecov.io/gh/pandas-dev/pandas/pull/23474?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Not for this PR, but a couple of other patterns that would be relatively easy to lint for:

`cimport numpy as np` --> should be `cnp` (not a widely-used convention, but it should be)
`*` imports
number of lines after function/class

For these purposes (and for this PR) we just assume away comments and docstrings. Thanks @jbrockmendel, nice addition.

There are reasons for having it with both the linting and the patters, happy with any.