Enable Mypy parallel type-checking

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.

Start with the linked mypy parallel type-checking documentation and review the project’s current mypy configuration. Check whether parallel checking and --native-parser have exited their experimental phase; done means the project can enable and recommend the option with confidence, but the issue does not identify a file or test to change.

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