Replaced `os.path` with `pathlib`

[trag1c]

Hey, I just made a Pull Request!

Description

Changes all occurrences of os.path to use pathlib instead.

Motivation and Context

Closes #179

Have you tested this? If so, how?

Updated unit tests :)

Checklist for PR author(s)

• [x] Changes are covered by unit tests (no major decrease in code coverage %).
• [x] All tests pass.
• [x] Docstring coverage is 100% via tox -e docs or interrogate -c pyproject.toml (I mean, we should set a good example :smile:).
• [ ] Updates to documentation:
  • [x] Document any relevant additions/changes in README.rst.
  • [x] Manually update both the README.rst and docs/index.rst for any new/changed CLI flags.
  • [ ] Any changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in the project's __init__.py file.

Release note

NONE

[trag1c]

Not sure if this deserves a release note or any versionchanged directives.

I'll take a look at Windows' fails tomorrow :+1:

[trag1c]

I'll take a look at Windows' fails tomorrow 👍

Took a bit longer but I'm getting OSErrors, seems like Windows can't install cairosvg 🤔 I feel like I'm doing something wrong here.

@econchick Could you approve the workflow? 😄