python: Re-lock pipenv at *oldest* supported versions

tox is already testing the most recent versions. Let's use pipenv to
test the oldest versions we claim to support. This matches the stylistic
choice to have pipenv always test our oldest supported Python version, 3.6.

The effect of this is that the python-check-pipenv CI job on gitlab will
now test against much older versions of these linters, which will help
highlight incompatible changes that might otherwise go unnoticed.

Update instructions for adding and bumping versions in setup.cfg. The
reason for deleting the line that gets added to Pipfile is largely just
to avoid having the version minimums specified in multiple places in
config checked into the tree.

(This patch was written by deleting Pipfile and Pipfile.lock, then
explicitly installing each dependency manually at a specific
version. Then, I restored the prior Pipfile and re-ran `pipenv lock
--dev --keep-outdated` to re-add the qemu dependency back to the pipenv
environment while keeping the "old" packages. It's annoying, yes, but I
think the improvement to test coverage is worthwhile.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-id: 20210629214323.1329806-5-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>