textproc/py-bracex: someone decided it was illegal to set both PORTVERSION and DISTVERSION

So, instead of a relatively neat and simple setting of two variables,
you now get this monstrosity. Replicate chunks of
../Mk/Uses/python.mk substituting our own PEP440VERSION rather than
DISTVERSION. We can't just set DISTVERSION because that breaks the
PEP440 versioning scheme where `2.5.post1' > '2.5' but the usual
DISTVERSION -> PORTVERSION conversion creates '2.5.p1' which is <
'2.5'. In this case we want a PORTVERSION of '2.5p1' > '2.5'