Add a Sphinx extension to document cached properties of slots classes

[clayote]

Summary

This is a small extension for Sphinx, the documentation generator that attrs uses. Where cached properties on slots classes were missing previously (#1325), this extension restores them.

I added a dictionary to slotted class objects, __attrs_cached_properties__, where they keep the cached property objects for Sphinx.

To use the extension, add "attrs.sphinx_cached_property" to extensions in your Sphinx conf.py.

Pull Request Check List

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• Added tests for changed code.
  Our CI fails if coverage is not 100%.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file typing-examples/baseline.py or, if necessary, typing-examples/mypy.py.
  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
• Updated documentation for changed code.
  • New functions/classes have to be added to docs/api.rst by hand.
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    The next version is the second number in the current release + 1.
    The first number represents the current year.
    So if the current version on PyPI is 22.2.0, the next version is gonna be 22.3.0.
    If the next version is the first in the new year, it'll be 23.1.0.
    • If something changed that affects both attrs.define() and attr.s(), you have to add version directives to both.
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[clayote]

I wrote a test for this, but it won't run in CI currently, because that test requires Sphinx to be installed. That's why the pre-commit check is failing.