Add BytesIO alias to fix missing references in docs

[nulano]

A warning has started appearing in the Docs build in main due to type hints added in #7727 and #7728.

/opt/hostedtoolcache/Python/3.12.1/x64/lib/python3.12/site-packages/PIL/GdImageFile.py:docstring of PIL.GdImageFile.open:1: WARNING: py:class reference target not found: _io.BytesIO /opt/hostedtoolcache/Python/3.12.1/x64/lib/python3.12/site-packages/PIL/MpegImagePlugin.py:docstring of PIL.MpegImagePlugin.BitStream:1: WARNING: py:class reference target not found: _io.BytesIO 

This PR fixes it using autodoc_type_aliases.

[hugovk]

io.BytesIO is incorrectly converted to _io.BytesIO

Do you know where this is coming from?

[nulano]

io imports BytesIO from the _io module where it is defined using the full name _io.BytesIO: https://github.com/python/cpython/blob/e6495159f6f2ca4fe3b5143c62fc2ad5873ef54f/Lib/io.py#L55-L56
https://github.com/python/cpython/blob/e6495159f6f2ca4fe3b5143c62fc2ad5873ef54f/Modules/_io/bytesio.c#L1057

This name can be observed from the console:

Python 3.11.4 (tags/v3.11.4:d2340ef, Jun 7 2023, 05:45:37) [MSC v.1934 64 bit (AMD64)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> import io >>> io.BytesIO <class '_io.BytesIO'>

However, it is listed under io.BytesIO in the documentation (note the url):
https://docs.python.org/3/library/io.html#io.BytesIO

I'm not sure if this would qualify as a CPython bug, a docs.python.org bug, or a sphinx autodoc bug.

[hugovk]

I wonder what's changed for it to start happening now? The last Python releases were in December, and Sphinx was in September. The direct docs dependencies also haven't recently changed. It feels like something that should be fixed upstream, somewhere, if possible...

[nulano]

I wonder what's changed for it to start happening now?

These references are in type hints added in #7727 and #7728.

[radarhere]

Suggested change

	"BytesIO": "~io.BytesIO",
	"BytesIO": "io.BytesIO",

I find that the tilde isn't necessary? radarhere@f698cb9 / https://github.com/radarhere/Pillow/actions/runs/7599204419/job/20695972684

[nulano]

The tilde is for consistency with other types.
With the tilde:

Without the tilde:

Compare with e.g. typing.BinaryIO:

[radarhere]

Closing, in favour of #7750