CLN: remove __unicode__ from pyx, etc. files
- [x] xref #25725

Removes use of ``__unicode__`` from the all pyx, pxi pxi.in etc type files. 

This pattern was almost not used in the non-.py type files.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26416?src=pr&el=h1) Report
> Merging [#26416](https://codecov.io/gh/pandas-dev/pandas/pull/26416?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ff4437eb3b27440ea2304a8613369ebec0b8b72a?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26416/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26416?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26416      +/-   ##
==========================================
- Coverage   91.69%   91.68%   -0.01%     
==========================================
  Files         174      174              
  Lines       50739    50739              
==========================================
- Hits        46524    46520       -4     
- Misses       4215     4219       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.19% <ø> (ø)` | :arrow_up: |
| #single | `41.18% <ø> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26416?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26416/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26416/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26416?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/26416?src=pr&el=footer). Last update [ff4437e...d965f5d](https://codecov.io/gh/pandas-dev/pandas/pull/26416?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/26416?src=pr&el=h1) Report
> Merging [#26416](https://codecov.io/gh/pandas-dev/pandas/pull/26416?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ff4437eb3b27440ea2304a8613369ebec0b8b72a?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26416/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26416?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26416      +/-   ##
==========================================
- Coverage   91.69%   91.68%   -0.01%     
==========================================
  Files         174      174              
  Lines       50739    50739              
==========================================
- Hits        46524    46520       -4     
- Misses       4215     4219       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.19% <ø> (ø)` | :arrow_up: |
| #single | `41.18% <ø> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26416?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26416/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26416/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26416?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/26416?src=pr&el=footer). Last update [ff4437e...d965f5d](https://codecov.io/gh/pandas-dev/pandas/pull/26416?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 The MacOs error in a connection issue, so unrelated. Restarted that to be safe. Do we have tests that hit this? IIRC the semantics for Cython don't 100% follow Python when it comes to str / unicode handling even with the language_level being set to 3 in our configuration, so would be good to double check these I suspect there won't be any strange Cython interaction. `__unicode__` was just a pandas thing. It doesn't have any special meaning in python / Cython. thanks @topper-123 