Replace distutils.LooseVersion by Version

Every import of distuils now raises a warning, and the module is
slated for removal.

The recommended replacement is packaging.version. It has Version and
LegacyVersion. LegacyVersion documents itself as "This class
implements the previous de facto sorting algorithm used by
setuptools". (Comparisons of packaging.LegacyVersion and
packaging.Version are intentionally borked, so packaging.LegacyVersion
should not be mixed with packaging.Version or packaging.parse(), which
can return packaging.Version.) But use of LegacyVersion also raises
a deprecation warning, with removal slated for the next release.

The runtime requirement on 'distutils' is dropped, since it is not
needed any more.