Update dependencies

Pytest-asyncio is once again making breaking changes that cause a bunch 
of warnings to be raised, some from the plugin itself, so we just pin it 
to the working version for now.

Pipfile

@@ -34,7 +34,7 @@ flaky = "*"
 hypothesis = "*"  # Versions between 6.47.1 and 6.56.4 added a prerelease dependency. See https://github.com/pypa/pipenv/issues/1760
 pdoc3 = "*"
 pytest = "*"
-pytest-asyncio = "*"
+pytest-asyncio = "~=0.21.0"
 pytest-cov = "*"
 pytest-mock = "*"
 vulture = "*"