Support PyPy builds

[milesgranger]

Adds PyPy36 support.

• Needs to run pypy_patch.py after builds until PyPy: Module works, but wheels can't be installed/develop doesn't install in virtualenv PyO3/maturin#312 is closed
• Cannot support Windows PyPy until Full PyPy support  PyO3/maturin#115 is closed

[milesgranger]

@martindurant is this helpful at all for our conda recipe issues? Just missing Window's PyPy...

[martindurant]

Good enough for me! I'm not going to stretch too far for pypy, especially since fastparquet, the first user of cramjam, dopesn't run on pypy anyway (because of numba).

[martindurant]

As far as conda-forge is concerned, let's release a new version with the allocation updates sooner rather than later. Perhaps should open a new issue of things we'd like to see happen by that release.

[milesgranger]

Great, was going to merge this then #26 if you're good with that.
Is it also okay with a 2.0.0-rc2 prerelease, as would still like to do some minor housekeeping; although admittedly nothing that would affect the major/minor version I guess.

[martindurant]

Is it also okay with a 2.0.0-rc2 prerelease

I've never done this. For pydata packages, most just inc the smallest number as seems fit. We don't strictly semantic-version, and indeed many are coving to calendar-version to avoid any confusion.