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.
To remove fixtures.py::add_funcargs_pseudo_fixture_def and add its logic
i.e. registering funcargs as params and making corresponding fixturedefs,
right to Metafunc.parametrize in which parametrization takes place.
To remove funcargs from metafunc attributes as we populate metafunc
params and make pseudo fixturedefs simultaneously and there's no need to
keep funcargs separately.