Replace os.path with pathlib

[seisman]

pathlib is the standard and preferred way to manipulate paths. We should migrate os.path calls to pathlib.

References:

• https://docs.python.org/3.8/library/pathlib.html
• https://treyhunner.com/2018/12/why-you-should-be-using-pathlib/

[seisman]

Migrating from os.path to pathlib.Path also help testing the pathlib support of PyGMT, but before making the changes, we need to address #1912 first.

[seisman]

We probably want to enable ruff's PTH rules after we finish the replacements.