CLN: use float64_t consistently instead of double, double_t
remove some commented-out or otherwise unused code

disable boundscheck/wraparound in a couple places in tslib where it is safe

add `const` modifiers to some of the memoryview functions so they don't raise if we ever pass read-only arrays to them

standardized NPY_NAT as always being the `cdef int64_t` version and `iNaT` as being the python-namespace version

remove duplicated NaN/nan constants

remove non-standard imports of np.nan in some test files
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/arrays/sparse/test_array.py`](https://github.com/jbrockmendel/pandas/blob/8bc71b8617f811bd7fc9dfa75cba58773415abc1/pandas/tests/arrays/sparse/test_array.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_operators.py`](https://github.com/jbrockmendel/pandas/blob/8bc71b8617f811bd7fc9dfa75cba58773415abc1/pandas/tests/frame/test_operators.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_repr_info.py`](https://github.com/jbrockmendel/pandas/blob/8bc71b8617f811bd7fc9dfa75cba58773415abc1/pandas/tests/frame/test_repr_info.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_timeseries.py`](https://github.com/jbrockmendel/pandas/blob/8bc71b8617f811bd7fc9dfa75cba58773415abc1/pandas/tests/frame/test_timeseries.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_to_csv.py`](https://github.com/jbrockmendel/pandas/blob/8bc71b8617f811bd7fc9dfa75cba58773415abc1/pandas/tests/frame/test_to_csv.py) !

 - There are no PEP8 issues in the file [`pandas/tests/groupby/aggregate/test_cython.py`](https://github.com/jbrockmendel/pandas/blob/8bc71b8617f811bd7fc9dfa75cba58773415abc1/pandas/tests/groupby/aggregate/test_cython.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_operators.py`](https://github.com/jbrockmendel/pandas/blob/8bc71b8617f811bd7fc9dfa75cba58773415abc1/pandas/tests/series/test_operators.py) !

 Closes #23371 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23583?src=pr&el=h1) Report
> Merging [#23583](https://codecov.io/gh/pandas-dev/pandas/pull/23583?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8ed92efc65b51a9010bd14e0e666fc722226e50a?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/23583/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23583?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23583   +/-   ##
=======================================
  Coverage   92.25%   92.25%           
=======================================
  Files         161      161           
  Lines       51277    51277           
=======================================
  Hits        47305    47305           
  Misses       3972     3972
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <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/23583?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/23583?src=pr&el=footer). Last update [8ed92ef...23344b2](https://codecov.io/gh/pandas-dev/pandas/pull/23583?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks. does isort work for .pyx? if so can you create an issue to enable it? did you need to remove anything from setup.cfg? >  does isort work for .pyx?

nope, actually mangles cimports pretty badly

> did you need to remove anything from setup.cfg?

no