Add follow_symlink argument to rmdir()

[hagenw]

This adds the follow_symlink argument to audeer.rmdir(), but sets it to True as default as this is avoids breaking changes and seems to be a meaningful behavior.
I decided against silently setting it to True internally as we still use audeer.rmdir() in loops that can get a speed up by ensuring that we don't have a symbolic link before we call audeer.rmdir(), e.g. https://github.com/audeering/audb/blob/12ad8520e45c11f152c5c96b11f221e3a1fb40f2/audb/core/load_to.py#L303-L329

I also added tests and included a new error under Raises section in the docstring that can happen with follow_symlink=False.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e6c534e) 100.0% compared to head (0b6e179) 100.0%.

Additional details and impacted files

Files	Coverage Δ
audeer/core/io.py	100.0% <100.0%> (ø)