gitlab-ci: provide python 3.6 and 3.8 for testing with tox

Also, moved the python 3.7 tox test to the same pattern than 3.6 and 3.8.

We don't need to use the built image for testing with tox since it uses a virtual env on its own.

Signed-off-by: Andres Gomez agomez@igalia.com