Try building on Apple M1

[ev-br]

Probably need:

• numpy >= 1.21
• -falign-functions=8? (https://stackoverflow.com/questions/65745683/how-to-install-scipy-on-apple-silicon-arm-m1)
• guard sys.platform in setup.py

[ev-br]

Apparently, it just works with numpy 1.21 -- checked by M. Sh.
09c8f60 documents the requirement.