Mk/Uses/python.mk: Support PEP440 versioning

The python PEP440 version numbering standard is _mostly_ compatible
with FreeBSD port versioning rules. Exceptions exist, where the
PORTVERSION can be derived from the upstream DISTVERSION
automatically. For example:

PEP440 DISTVERSION: FreeBSD PORTVERSION:
2.3.post1 2.3.p1

Now, this interacts badly with PEP517 build setups. hatchling will
enforce PEP440 complicance, so it isn't practical to modify the ported
code to use exactly the FreeBSD version.

Instead, simply referring to DISTVERSION rather than PORTVERSION will
allow the build process to complete smoothly.

See https://reviews.freebsd.org/D39123 for an example port update
which depends on this change

Approved by: python (maintainer, vishwin)
Differential Revision: https://reviews.freebsd.org/D39124