Update autodoc defaults (#3369)

Looking between the documentation for `v2.7.0` and `v2.8.0`, all member
function documentation has vanished, making it extremely difficult to
utilize the documentation and for new users to be acquainted with the
library. This appears to be from a [change in
Sphinx](https://www.sphinx-doc.org/en/master/extdev/deprecated.html#:~:text=autodoc_default_flags,autodoc_default_options)
deprecating `autodoc_default_flags` in favor of
`autodoc_default_options`, which was exposed when the Sphinx version was
bumped (6f0d6efaebb40ce78b961ca84cd494b103d37894) in
`requirements/docs.txt`. Let me know if I'm missing anything!

Fixes #3373.

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>