mypy: disallow untyped defs by default (#11862)

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.

testing/test_python_path.py

@@ -1,3 +1,4 @@
+# mypy: allow-untyped-defs
 import sys
 from textwrap import dedent
 from typing import Generator