Fix AttributeError when importing on Python 3.14

[hugovk]

Summary

Fixes #995.

On Python 3.14, we get an AttributeError on import:

❯ python3.14
Python 3.14.0a7 (v3.14.0a7:29af6cee02f, Apr  8 2025, 07:53:42) [Clang 16.0.0 (clang-1600.0.26.6)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import httpcore
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    import httpcore
  File "/private/tmp/httpcore/httpcore/httpcore/__init__.py", line 140, in <module>
    setattr(__locals[__name], "__module__", "httpcore")  # noqa
    ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'typing.Union' object has no attribute '__module__' and no __dict__ for setting new attributes. Did you mean: '__reduce__'?

This is caused by SOCKET_OPTION, let's exclude it:

❯ python3.14
Python 3.14.0a7 (v3.14.0a7:29af6cee02f, Apr  8 2025, 07:53:42) [Clang 16.0.0 (clang-1600.0.26.6)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import httpcore
>>>

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[zanieb]

A comment would be good.

[lovelydinosaur]

Thanks @hugovk, @zanieb.

A release PR from this point would make sense.

[hugovk]

Please see PR #1006.