chore: fix issue if only run test_gitlab.py func test

The GitHub CI is showing a ResourceWarning, causing our test to fail.

Update test to only look for our UserWarning which should not appear.

What was seen when debugging the GitHub CI:
{message:
    ResourceWarning(
        "unclosed <socket.socket fd=12, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('127.0.0.1', 50862), raddr=('127.0.0.1', 8080)>"
    ),
    category: 'ResourceWarning',
    filename: '/home/runner/work/python-gitlab/python-gitlab/.tox/api_func_v4/lib/python3.10/site-packages/urllib3/poolmanager.py',
    lineno: 271,
    line: None
}

Make it so can run just the test_gitlab.py functional test.

For example:
$ tox -e api_func_v4 -- -k test_gitlab.py

chore(deps): update pre-commit hook pycqa/flake8 to v5