CLN: Remove PY2/3 checks in cython files
- [x] xref #25725
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Remove PY2/3 checks in `pyx` files. 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25876?src=pr&el=h1) Report
> Merging [#25876](https://codecov.io/gh/pandas-dev/pandas/pull/25876?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/324bb84c8e4c84b4103e9b2bdda2d99e8ef549db?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/25876/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25876?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25876   +/-   ##
=======================================
  Coverage   91.48%   91.48%           
=======================================
  Files         175      175           
  Lines       52885    52885           
=======================================
  Hits        48380    48380           
  Misses       4505     4505
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.04% <ø> (ø)` | :arrow_up: |
| #single | `41.82% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25876?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/25876?src=pr&el=footer). Last update [324bb84...de2414b](https://codecov.io/gh/pandas-dev/pandas/pull/25876?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25876?src=pr&el=h1) Report
> Merging [#25876](https://codecov.io/gh/pandas-dev/pandas/pull/25876?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/324bb84c8e4c84b4103e9b2bdda2d99e8ef549db?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/25876/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25876?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25876   +/-   ##
=======================================
  Coverage   91.48%   91.48%           
=======================================
  Files         175      175           
  Lines       52885    52885           
=======================================
  Hits        48380    48380           
  Misses       4505     4505
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.04% <ø> (ø)` | :arrow_up: |
| #single | `41.82% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25876?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/25876?src=pr&el=footer). Last update [324bb84...2716296](https://codecov.io/gh/pandas-dev/pandas/pull/25876?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @mroeschke 