Mypy tests are slow.

[hirosassa]

Mypy tests are slow.
Need to be fixed.

10.23s call     test/test_mypy.py::TestMyMypyPlugin::test_plugin_no_issue
10.21s call     test/test_mypy.py::TestMyMypyPlugin::test_parameter_has_default_type_invalid_pattern
10.20s call     test/test_mypy.py::TestMyMypyPlugin::test_plugin_invalid_arg
10.14s call     test/test_mypy.py::TestMyMypyPlugin::test_parameter_has_default_type_no_issue_pattern
0.00s setup    test/test_mypy.py::TestMyMypyPlugin::test_parameter_has_default_type_invalid_pattern
0.00s setup    test/test_mypy.py::TestMyMypyPlugin::test_plugin_invalid_arg
0.00s setup    test/test_mypy.py::TestMyMypyPlugin::test_plugin_no_issue
0.00s setup    test/test_mypy.py::TestMyMypyPlugin::test_parameter_has_default_type_no_issue_pattern
0.00s teardown test/test_mypy.py::TestMyMypyPlugin::test_plugin_no_issue
0.00s teardown test/test_mypy.py::TestMyMypyPlugin::test_parameter_has_default_type_no_issue_pattern
0.00s teardown test/test_mypy.py::TestMyMypyPlugin::test_plugin_invalid_arg
0.00s teardown test/test_mypy.py::TestMyMypyPlugin::test_parameter_has_default_type_invalid_pattern

[hirosassa]

Most of the time is consumed by mypy.api.run.

[hirosassa]

From my local experiments, I found that mypy's cache utilization is slightly slower than pytest-xdist's parallelization.
The cache utilization strategy needs serial processing through mypy tests.
That overhead is larger than speed up by using cache.