Broken link in 0020

@dvzrv The link on the following line is broken. I could not find the updated link. It may have been removed from docs.python.org:

https://gitlab.archlinux.org/archlinux/rfcs/-/blob/ff5e907a299efa56cc57c91156427660b695ef46/rfcs/0020-sources-for-python-packaging.rst#L61