Can we require CFFI for PURE_PYTHON usage on CPython?

[jamadden]

CFFI distributes binary wheels, so it's a lightweight dependency.

This would simplify our code by letting us remove one of the two implementations of picklecache.Ring, and it would simplify our test matrix and help us reach 100% coverage.

[jamadden]

Also, with #45, I don’t want to have to implement it three times.

[jimfulton]

+1. If it blows up in some unexpected way we can revert. :)

[jamadden]

CFFI is not available for Jython, so this would eliminate support for that platform. We don't actually test Jython or list it in the classifiers, but there are some allowances for it in the code (which I would remove as part of this work).

Knowing that, is it still a +1?

[icemac]

+1 from me.