[internal] Replace`os.path` with `Pathlib`

[17o2]

Remove all references to os.path in the code and replace with the equivalents using Pathlib for consistency and robustness.

• Source 1.1
• Source 1.2

This includes reducing all type hints from (str, Path) to Path.

[17o2]

Perhaps @n42 can explain the need for adding the .. in line 13 in wireviz.py? Isn't it enough to insert os.path.dirname(__file__) to find the WireViz modules?

Would the Pathlib equivalent then be Path(__file__).parent? Thanks :)