Fix bib resource finding, subprocess call, and upcoming deprecation

[tobiscode]

Hi, as discussed in https://github.com/twilsonco/latexdiffcite/issues/12, I've forked the repo and applied my changes. It turned out I had to do one other fix to debug my current installation, so it's now a slightly bigger PR. In detail:

• 00e0905630ee2b8f8436d2f566dadf71e7c77063 fixes https://github.com/twilsonco/latexdiffcite/issues/12
• ebcf25728d6fbae8678d1b49644abe9d126d7479 fixes https://github.com/twilsonco/latexdiffcite/issues/9
• 3a0727bdc58b48255709c461df7342d78b41a286 fixes upcoming deprecation https://docs.python.org/3/whatsnew/3.12.html#other-language-changes

I'm merging it into the develop branch as requested in the guidelines even though it's a bit behind on commits, maybe that should be changed to the main branch. I also haven't run any tests yet since I made the commits from a non-TeX machine and the tox setup also seemed pretty outdated. @twilsonco if you could run the tests that'd be amazing, then we would also have a second check, but if not, I can do them sometime in the future.

Cheers

[twilsonco]

Wow. Thanks for putting this together. I haven't tested yet, but the next time I use this (for a paper revision due in 4 weeks) I'll be using this version.

Merging.