If we do this (I still need to look at the details), instead of the max cpu count we should use the -j option which is stored as self.parallel in ExtensionBuilder.

re: configurability
I don't mind at all if you take over this PR :)

re: automated checks...
I see that Python 2.7 and 3.5 checks are still configured for this project, however, the only wheels are only available for 3.6 ~ 3.9 on PYPI.
I'm not sure what Python versions should ultimately be supported.
I think that f-string is the only "new" feature in the proposed change.

We have a wider range of python version compatibility than published wheels in a lot of cases, for instance we've kept spacy v2.x compatible with python 2.7 even though we haven't published python 2.7 wheels for a while.

I have to look a bit more into the details about what kind of multiprocessing setup makes sense, since I'd rather have slow builds than broken/brittle builds.

And probably it does make sense to do this in combination with an environment variable so this also works with pip and other build systems, something like numpy's NPY_NUM_BUILD_JOBS.

This has probably gotten stale, feel free to close.