Fix functional tests

Limited pytest to <5.4.0 due to pytest-dev/pytest-asyncio#141.

This interim step enables functional testing to run, until LP:#1890367
is resolved.

src/tests/functional/requirements.txt

@@ -1,6 +1,6 @@
 flake8
 juju
 mock
-pytest
+pytest < 5.4.0  # pytest 5.4.0 deprecated `pytest.Function()` construction
 pytest-asyncio == 0.10.0 # https://github.com/pytest-dev/pytest-asyncio/issues/154
 requests