Fix installing on PyPy

[mgorny]

Remove the incorrect assumption that PyPy does not use the extension modules -- it needs them just the same as other Python implementations. Make the cffi dependency specific to CPython, as it is vendored in PyPy. Fail if CFFI cannot be imported, as otherwise broken package will be installed.

Fixes #397

[jnwatson]

Didn't work (see above)

[mgorny]

I'm sorry to say but it only means it never worked in the first place, because the error was silently ignored before, producing a nonfunctional package that only passed CI because the source directory was still around.

That said, I'm fine with restoring the code to ignore errors and produce broken packages while pretending everything's fine, as long as the PyPy fix is in and we at least try to produce a working package instead of deliberately producing a broken one.