Enable Mypy parallel type-checking #355

Allow this (and the --native-parser underneath it) to mature and exit experimental phase and then recommend enabling it

• https://mypy.readthedocs.io/en/stable/command_line.html#parallel-type-checking

By default, mypy checks all modules in the same Python process. This can be slow for large code bases. Mypy offers experimental parallel type-checking mode using multiple worker processes.