* Add flags to pass on --warn-unused-ignores and --no-implicit-optional to mypy
* Make implicit Optional explicit in arg types (2and3 part)
* Convert {stdlib,third_party}/2 to explicit Optional
- Moves the pdb stub from separate `2`/`3` files to a single file in `2and3`, as they were identical.
- Maintains the comment header, as the stub is incomplete.
