In this PR, all usages of abspath, basename, dirname, exists, isdir, join, isfile, splittext... from os.path are replaced with Path from pathlib. A few resources from os.path are kept (getmtime, relpath, splitdrive, commonprefix).

The modifications in this PR should NOT introduce any breaking change, as I tried to keep internal data types (strings).

In the second commit, some type annotations are added to vunit/ui/__init__.py.

Actually, this PR is a cleaning phase before properly using Path internally. Future work should consist on evaluating when explicit conversions to str can be avoided. I believe this should be done together with adding type annotations, as it allows to better catch inconsistencies.