python: Configure tox to skip missing interpreters

python: Configure tox to skip missing interpreters

Currently tox tests against the installed interpreters, however if any
supported interpreter is absent then it will return fail. It seems not
reasonable to expect developers to have all supported interpreters
installed on their systems. Luckily tox can be configured to skip
missing interpreters.

This changed the tox setup so that missing interpreters are skipped by
default. On the CI, however, we still want to enforce it tests
against all supported. This way on CI the
--skip-missing-interpreters=false option is passed to tox.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20210630184546.456582-1-wainersm@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>