Fix sphinx extension for newer versions of sphinx

[hagenw]

Closes #99

sphinx.application.Sphinx.srcdir does no longer return a string, but an _StrPat object in newer versions of sphinx.

This adds tests for newer versions of sphinx, by avoiding using our sphinx-audeering-theme in the tests as it only work with sphinx<6.0.

[ChristianGeng]

This makes the packages's sphinx app future-proof by fixing a typing issue.
The tests run against the alabaster theme as sphinx-audeering-theme cannot support more recent sphinx-versions.

With limited access to internet I cannot test locally too well for now.
But:

I can see that all actions for the sphinx matrix are passsing:

sphinx: [ 'sphinx==5.3.0', 'sphinx==6.2.1', 'sphinx==7.*' ]

I also see that the replace methods in sphinx.__init__ should be safe as it only has an effect if the input type is not a string.

I therefore believe that this is safe to approve.