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.
* Rename pytest_ignore_collect fspath parameter to collection_path
* Rename pytest_collect_file fspath parameter to file_path
* Rename pytest_pycollect_makemodule fspath parameter to module_path
* Rename pytest_report_header startpath parameter to start_path
* Rename pytest_report_collectionfinish startpath parameter to start_path
* Update docs with the renamed parameters
* Use pytest-flakes fork temporarily to prove it works
* Use pytest-flakes 4.0.5
