Update links in reference implementation section

Author: astrofrog

peps/pep-0771.rst

@@ -354,26 +354,25 @@ properly document the options available to users in terms of installation.
 Reference Implementation
 ========================
 
-The following branch contains a modified version of the `Flit
-<https://flit.pypa.io/en/stable/>`_ package which recognizes the
-``default-optional-dependencies`` key in ``pyproject.toml`` and adds the
-relevant ``Default-Extra:`` metadata to the build package:
+The following repository contains a fully functional demo package
+that makes use of default extras:
 
-https://github.com/astrofrog/flit/tree/default-extras-pep
+https://github.com/wheel-next/pep_771
 
-In addition, the following branch contains a modified version of `pip
-<https://pip.pypa.io/en/stable/>`_ which implements the correct logic for
-installing extras when default extras are present:
+This makes use of modified branches of several packages, and the following
+links are to these branches:
 
-https://github.com/astrofrog/pip/tree/default-extras-pep
+* `Setuptools <https://github.com/wheel-next/setuptools/tree/pep_771>`_
+* `pip <https://github.com/wheel-next/pip/tree/pep_771>`_
+* `importlib_metadata <https://github.com/wheel-next/importlib_metadata/tree/pep_771>`_
 
-Finally, the following repository makes use of the above two branches to
-test out different scenarios and use cases described in this PEP:
+In addition, `this branch <https://github.com/astrofrog/flit/tree/default-extras-pep>`_
+contains a modified version of the `Flit
+<https://flit.pypa.io/en/stable/>`_ package.
 
-https://github.com/astrofrog/pep-771-demo
 
-The implementations above are proofs-of-concept at this time, do not include any
-tests or documentation in the respective packages, and the existing changes have
+
+The implementations above are proofs-of-concept at this time and the existing changes have
 not yet been reviewed by the relevant maintainers. Nevertheless, they are
 functional enough to allow for interested maintainers to try these out.