Filter out None results before sorting python version list

[Systemcluster]

This solves the issue of trying to sort non-python results in the find_all_python_versions function.

This was attempted in #87, but this is still an issue for me and a lot of other users.

For some reason, the versions list can contain non-existing entries. For instance on my machine it contains among others an entry of a version I uninstalled a long time ago:

PathEntry(path=WindowsPath('C:/Users/Chris/AppData/Local/Programs/Python/Python37/python.exe'), _children={}, only_python=True, name='3.7', _py_version=None, _pythons=defaultdict(None, {}), _is_dir=False, _is_executable=False, _is_python=False, is_root=False), 

When as_python.version_sort of such an entry is accessed in order to sort the list, it panics because as_python returns None.

It would probably be best to attack this issue at the source as well, but filtering out these kind of results at the location where they're accessed is a start.

[Systemcluster]

I just noticed that 795fe9a tackled this issue as well (I think?), so this might not be needed anymore. Should this still be checked at this location to guard against regressions or other sources of invalid entries?

[Systemcluster]

Any interest in merging this @frostming?