docs: How to use requests-cache together with gitlab?

This shows how to use reclosedev/requests-cache and could be added to the FAQs. Want a PR?

import gitlab as glapi

GITLAB_URL = "https://…"

class GitLab(glapi.Gitlab):
    """ GitLab API.

        * https://python-gitlab.readthedocs.io/en/stable/api/gitlab.html
        * https://docs.gitlab.com/ce/api/#resources
    """

    API_BASEURL = GITLAB_URL
    CACHE_EXPIRATION = 2 * 60 * 60  # seconds

    def __init__(self):
        account, token = security.Credentials(self.API_BASEURL).auth_pair()
        session = requests_cache.CachedSession(
            cache_name=config.cache_file(type(self).__name__),
            expire_after=self.CACHE_EXPIRATION)
        with session.cache_disabled():
            super().__init__(self.API_BASEURL, private_token=token, session=session)

    def auth(self):
        """ Performs a non-cached authentication.
        """
        with self.session.cache_disabled():
            return super().auth()

@jhermann Sorry for the late response. Yes, we're always open for merge requests, but I'm not sure that the docs would be in scope for python-gitlab. I'm afraid that it could confuse people, maybe?