Fix functional test failure if config present and configured with
token.

Closes: #1791

Ensure reset_gitlab() succeeds by waiting to make sure everything has
been deleted as expected. If the timeout is exceeded fail the test.

Not using `wait_for_sidekiq` as it didn't work. During testing I
didn't see any sidekiq processes as being busy even though not
everything was deleted.

chore: skip a functional test if not using >= py3.9

One of the tests requires Python 3.9 or higher to run. Mark the test
to be skipped if running Python less than 3.9.

When running with docker-compose on Ubuntu 20.04 I got the error:

  $ docker-compose up
  ERROR: The Compose file './docker-compose.yml' is invalid because:
  networks.gitlab-network value Additional properties are not allowed ('name' was unexpected)

Changing the version in the docker-compose.yml file fro '3' to '3.5'
resolved the issue.

BREAKING CHANGE: A config file is no longer needed to run
the CLI. python-gitlab will default to https://gitlab.com
with no authentication if there is no config file provided.
python-gitlab will now also only look for configuration
in the provided PYTHON_GITLAB_CFG path, instead of merging
it with user- and system-wide config files. If the
environment variable is defined and the file cannot be
opened, python-gitlab will now explicitly fail.

It is not yet available upstream.