Links for Cython

Note that for one-time builds, e.g. for CI/testing, on platforms that are not
covered by one of the wheel packages provided on PyPI *and* the pure Python wheel
that we provide is not used, it is substantially faster than a full source build
to install an uncompiled (slower) version of Cython with::

    pip install Cython --install-option="--no-cython-compile"

For newer pip versions with no "--install-option" support this has been
replaced by the more generic "--config-setting" option:

    pip install Cython --config-setting="--build-option=--no-cython-compile"