PyPy support

[hameerabbasi]

Feature request

As title. According to scikit-learn/scikit-learn#17364, scikit-learn (and PyData/Sparse) cannot support PyPy because Numba doesn't support it. Are there plans to support PyPy, and if not, are there reasons behind it?

I realize this may be a big undertaking as Numba operates on Python bytecode and I assume that bytecode is different with PyPy.

[gmarkall]

I've made this work once, but I never upstreamed it because getting it into an upstreamable state would have been quite a lot of work (it was a project I did in my free time) and without CI for PyPy I expected it to bitrot quite fast: https://www.embecosm.com/2017/01/19/running-numba-on-pypy/

[stuartarchibald]

xref numba/llvmlite#525

[gmarkall]

xref: #2319