cffi should specify entry points for PyPy too

Created originally on Bitbucket by daa

Currently setup.py specifies empty entry points when it's started by PyPy and consequenctly built and installed package does not have entry_points.txt in cffi.dist-info. So if one installs cffi into some directory and then points PYTHONPATH to it and then tries to build some package using cffi extension it will fail because setuptools won't be able to find extension for cffi_modules keyword. This happens because installed cffi.dist-info in PYTHONPATH is found first and pre-installed lib_pypy/cffi.egg-info is not used. For example pip-18 cannot build cryptography package with added pyproject.toml on PyPy, related issue is https://github.com/pypa/pip/issues/5631.