Change our mypy configuration to disallow untyped defs by default, which ensures *new* files added to the code base are fully typed.
To avoid having to type-annotate everything now, add `# mypy: allow-untyped-defs` to files which are not fully type annotated yet.
As we fully type annotate those modules, we can then just remove that directive from the top.
This will now issue a RemovedInPytest4Warning when the user calls
a fixture function directly, instead of requesting it from test
functions as is expected
Fix#3661