Run mypy with parallel workers

[hugovk]

mypy has added a --num-workers option:

Use NUMBER parallel worker processes (in addition to the coordinator process) to perform type-checking. Specifying --num-workers 0 (default) disables parallel checking. Automatic detection of the optimal number of workers is not supported yet.

https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-n

Here's running with some different values on an 8-core macOS:

• Cold cache = .mypy_cache deleted
• Warm cache = immediate re-run

--num-workers	Cold cache	Warm cache
(none / serial)	8.08	0.38
2	3.50	0.31
4	2.75	0.36
6	3.58	0.40
8	2.97	0.45
16	7.19	1.58

So let's go for 4.

This is mainly for local benefit, but GitHub Actions has 4 CPUs and also improves: from 20.60 seconds to 11.00 seconds.

---

I also sometimes have the extra test images installed, let's exclude them to avoid this:

Tests/images/sunraster/test.py:21: error: Function is missing a type annotation for one or
more parameters  [no-untyped-def]
    def test(f) -> None:
    ^~~~~~~~~~~~~~~~~~~~
Tests/images/picins/test.py:10: error: Function is missing a type annotation for one or more
parameters  [no-untyped-def]
    def test(f) -> None:
    ^~~~~~~~~~~~~~~~~~~~
Tests/images/msp/test.py:21: error: Function is missing a type annotation for one or more
parameters  [no-untyped-def]