gitlab: add python linters to CI

gitlab: add python linters to CI

Add a Python container that has just enough juice for us to run the
Python code quality analysis tools. Base this container on Fedora,
because Fedora has very convenient packaging for testing multiple Python
versions.

We need python3, pip (for pulling packages), pipenv and virtualenv for
creating virtual environments, and tox for running tests. make is needed
for running 'make check-tox' and 'make venv-check' targets. Python3.10
is needed explicitly because the tox package only pulls in 3.6-3.9, but
we wish to test the forthcoming release of Python as well to help
predict any problems. Lastly, we need gcc to compile PyPI packages that
may not have a binary distribution available.

Add two tests:

check-python-pipenv uses pipenv to test a frozen, very explicit set of
packages against our minimum supported python version, Python 3.6. This
test is not allowed to fail. The dependencies this test uses do not
change unless python/Pipfile.lock is changed.

check-python-tox uses tox to install the latest versions of required
python dependencies against a wide array of Python versions from 3.6 to
3.9, even including the yet-to-be-released Python 3.10. This test is
allowed to fail with a warning.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-id: 20210527211715.394144-32-jsnow@redhat.com
[Fix rebase conflict over .gitlab-ci.yml --js]
Signed-off-by: John Snow <jsnow@redhat.com>