PEP 384 introduced a Python limited API that allows you to build one
wheel that targets all Python versions on a given platform.

Setuptools supports opting in to building wheels for the Python
limited API by adding the following lines to ``setup.cfg``:

    [bdist_wheel]
    py_limited_api = cp36  # replace with desired min Python version

However, there are two needed steps that Setuptools does not do
automatically:

1. Pass the `py_limited_api=True` keyword argument to each `Extension`
2. Set the `Py_LIMITED_API` preprocessor macro to the version hex
   value for the desired minimum Python version

This patch teaches `get_extension()` to do those two missing steps
automatically if the limited API is enabled in the ``setup.cfg`` file.

Co-authored-by: Conor MacBride <conor@macbride.me>