Performance regression in pytest tests due to always writing cache

[JukkaL]

Commit 264715e slowed down pytest tests by about 50% when I run them locally. I'd rather not write cache files by default when running locally -- maybe we can only do it on CI builds. Alternatively, I'd like a documented option to skip writing cache to speed up tests.

[gvanrossum]

There's a documented option, --cache-dir=/dev/null (on Windows use nul instead -- the value must match os.devnull).

I think pytests are a special case, I don't see much slowdown on real runs (e.g. mypy mypy).

I'll look for a way to disable cache writing in all tests.