Python module rstcheck which is used by ansible-test deprecated
Python versions prior 3.7 in version 3.5.0 and removed support
in 4.0.0 [1]. Ubuntu 18.04 LTS (Bionic Beaver) comes with Python
3.6 by default, so rstcheck prints a FutureWarning which confuses
ansible-test:

  Run command: ***/python -m rstcheck --report warning
  --ignore-substitutions _,br,release,today,version
  docs/openstack_guidelines.rst
  Traceback (most recent call last):
    File "***/ansible-test", line 28, in <module>
      main()
    File "***/ansible-test", line 24, in main
      cli_main()
    File "***/ansible_test/_internal/cli.py", line 130, in main
      args.func(config)
    File "***/ansible_test/_internal/sanity/__init__.py", line 193,
    in command_sanity
      result = test.test(args, sanity_targets, version)
    File "***/ansible_test/_internal/sanity/rstcheck.py", line 80,
    in test
      results = parse_to_list_of_dict(pattern, stderr)
    File "***/ansible_test/_internal/util.py", line 799, in
    parse_to_list_of_dict
      raise Exception('Pattern "%s" did not match values:\n%s' %
      (pattern, '\n'.join(unmatched)))
  Exception: Pattern "^(?P<path>[^:]*):(?P<line>[0-9]+):
  \((?P<level>INFO|WARNING|ERROR|SEVERE)/[0-4]\) (?P<message>.*)$"
  did not match values:
  ***/rstcheck.py:51: FutureWarning: Python versions prior 3.7 are
  deprecated. Please update your python version.
    FutureWarning
  ERROR: Command "/usr/bin/env ANSIBLE_TEST_CONTENT_ROOT=***/
  ansible_collections/openstack/cloud LC_ALL=en_US.UTF-8 ***/python3.6
  ***/ansible-test sanity -v --python 3.6 --skip-test
  metaclass-boilerplate --skip-test future-import-boilerplate plugins/
  docs/ meta/ scripts/ --metadata ***.json --truncate 0 --redact
  --color no --requirements" returned exit status 1.

We cannot constrain the version of rstcheck which ansible-test installs
into its Python virtual environment. This has to be fixed in Ansible
itself, a pull request against Ansible 2.9 has been opened [2].

Ref.:
[1] https://github.com/myint/rstcheck/blob/master/README.rst
[2] https://github.com/ansible/ansible/pull/77568

As a workaround we temporarily set our Ubuntu 18.04 based linter job
openstack-tox-linters-ansible-2.9 to non-voting and remove it from
check dependencies and as a gate job.

Thanks to Arx Cruz and Jesper Schmitz Mouridsen for pointing out this
issue and its root cause ☺️

Change-Id: Ic6f2febc5a40a29534ac4c7f41f714865099086a