tests: use packaging from setuptools instead of deprecated distutils
When invoking StrictVersion in 3.12 we got:
DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead.
distutils is dead in the standard library, and we have to move towards using
`setuptools` as general extern dependency. Instead of also requiring the extern
`packaging`, we will just use the packaging that is vendored in setuptools.
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors