fix recently occurred type error in AbstractSlideBuilder

[lpirl]

Just experienced this

$ sphinx-build -M slides "." "build" -j auto -n -a
…
/…/slides/venv/lib/python3.11/site-packages/hieroglyph/builder.py:126: RemovedInSphinx80Warning: Sphinx 8 will drop support for representing paths as strings. Use "pathlib.Path" or "os.fspath" instead.
  doctree.attributes.get('source')[len(self.srcdir) + 1:].count('/')

Exception occurred:
  File "/…/slides/venv/lib/python3.11/site-packages/hieroglyph/builder.py", line 135, in post_process_images
    if node['uri'].startswith(self.outdir):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

and think the proposed change is good enough.

Thanks for considering.

[oliver-sanders]

Recently encountered this in my project, this patch seems to work 👍

[oliver-sanders]

This one can fail too:

Suggested change

	node['uri'][len(self.outdir) + 1:]
	node['uri'][len(str(self.outdir)) + 1:]

There is a third use of self..outdir in copy_static_files below but that should be fine:

>>> os.path.join('a', Path('b'), 'c')
'a/b/c'

[lpirl]

True, thanks.

By the way, of course all my PRs are available in https://github.com/lpirl/hieroglyph if that is of any interest to anyone.