Point pip's setup.py link to its latest version

This file no longer exists on the `main` branch as of
pypa/pip#12537 /
pypa/pip@0ad4c94b.

Author: webknjaz

source/guides/single-sourcing-package-version.rst

@@ -9,7 +9,8 @@ Single-sourcing the package version
 There are many techniques to maintain a single source of truth for the version
 number of your project:
 
-#.  Read the file in :file:`setup.py` and get the version. Example::
+#.  Read the file in :file:`setup.py` and get the version. Example (from `pip setup.py
+    <https://github.com/pypa/pip/blob/003c7ac/setup.py>`_)::
 
         import codecs
         import os.path