python: add vendor.py utility

python: add vendor.py utility

This is a teeny-tiny script that just downloads any packages we want to
vendor from PyPI and stores them in qemu.git/python/wheels/. If I'm hit
by a meteor, it'll be easy to replicate what I have done in order to
udpate the vendored source.

We don't really care which python runs it; it exists as a meta-utility
with no external dependencies and we won't package or install it. It
will be monitored by the linters/type checkers, though; so it's
guaranteed safe on python 3.6+.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-15-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>