static metadata

[dimbleby]

Revisits #97, actually more or less reverses 84650b2

per that issue, it still is desirable that package metadata should be static. That is not currently the case: some wheels say that they want cffi >= 1.17, while others say that they want cffi >= 1.11.

Meanwhile "cffi is required on PyPy." seems to be the opposite of reality. As noted earlier in that file, PyPy vendors cffi - so it is the one python implementation where you definitely do not need a dependency on cffi.