support pypy 3.8

[dand-oss]

pypy 3.8 is released for download.

pypy does not supply _pickle c extension

#455

pypy 3.8 should continue to import the pure python version of pickle.
pypy 3.7 worked because of the test to import pure python pickle if python version less than 3.7.

Check for pypy platform explicitly as fix.

[jrbourbeau]

Thanks @dand-oss -- this seems like a sensible change. There's a centralized PYPY variable already defined here

cloudpickle/cloudpickle/cloudpickle.py

Line 95 in d75566d

PYPY = platform.python_implementation() == "PyPy"

Would you mind using that here instead?

EDIT: In fact, it probably more sense to move the definition of the PYPY variable into the compat.py compatibility module and then import it the variable elsewhere when needed

[ogrisel]

Closing in favor of #461.