Add setuptools in pytest6

This should handle the import of `pkg_resources` which is required there.

Author: nicoddemus

tox.ini

@@ -9,6 +9,7 @@ deps =
     mock
     pytest-asyncio
     pytest6: pytest==6.2.5
+    pytest6: setuptools
 commands =
     coverage run --append --source={envsitepackagesdir}/pytest_mock -m pytest tests --color=yes