[functools] Add __slots__

[donbarbos]

Python 3.10: https://github.com/python/cpython/blob/3.10/Lib/functools.py
Changed PR: python/cpython#119827
Python 3.14: https://github.com/python/cpython/blob/3.14/Lib/functools.py

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[brianschubert]

Hmm, I don't think these actually have __slots__ at runtime. Both partial and _PlaceholderType are implemented in the _functools accelerator module, so the actual runtime types are C types which don't use __slots__:
https://github.com/python/cpython/blob/a7beca8ae3168f1bf191a6be5a2ee5a3009c88c2/Lib/functools.py#L438

>>> import functools
>>> functools.partial.__slots__
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    functools.partial.__slots__
AttributeError: type object 'functools.partial' has no attribute '__slots__'. Did you mean '.__class__' instead of '.__slots__'?