CI issues because sphinx dropped py37 support

[aseyboldt]

Description

See for instance
https://readthedocs.org/projects/pytensor/builds/19066465/
sphinx is importing functools.cached_property, which is new in py38.
I guess we could simply build the docs in >=py38?

[michaelosthege]

closed by #164