Fix PyPy dependency marker

[doe1080]

implementation_name maps to sys.implementation.name, whose value is guaranteed to be lowercase, so the marker should use pypy

https://packaging.python.org/en/latest/specifications/dependency-specifiers/#defined-environment-marker-fields
https://docs.python.org/3/library/sys.html#sys.implementation

[mattip]

PyPy ships its own version of cffi which cannot be replaced by one from pip at all. I wonder if there is a way to error out entirely if the pyproject.toml is ever run on PyPy.