Make the tests pass with Sphinx 7.2

[mitya57]

Starting with Sphinx 7.2, SphinxTestApp constructor expects srcdir to be pathlib.Path. Otherwise, the following error happens:

>       outdir.mkdir(parents=True, exist_ok=True)
E       AttributeError: 'path' object has no attribute 'mkdir'

[welcome]

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[agoose77]

Hi @mitya57, thanks for authoring this PR! I'm going to close it in favour of #6, because we've had to make these changes in other parts of the stack and I'm trying to keep them fairly identical.

But, your contribution is highly appreciated; if we hadn't had to make this change already, it would have been an immediate merge!

Thanks, and please don't hesitate to contribute again - we need these kinds of improvements! <3