Python Package Index (pypi) master site handling

[rodent%NetBSD.org@localhost]

Greetings pkgsrc{ers,resses},

I've added mk/fetch/pypi.mk to handle fetching distfiles from pypi
automatically. Upon enabling this file in mk/fetch/bsd.fetch.mk, we could just
use DISTNAME and MASTER_SITES=${MASTER_SITE_PYPI} in the pkgsrc Makefile and
variables like SITES.DISTNAME, HOMEPAGE, and PKGNAME would get handled
automatically.

It's based upon tnn@'s github.mk file and is something i've wanted to add for
a while. I put a couple hours of testing into it and it seems to work OK
without breaking anything. If it's all right, i'll uncomment the line in
bsd.fetch.mk and modify pkglint and url2pkg to be aware of this behaviour.

Best regards,

rodent@