Update pip for GitHub runner

This is to address an error seen on some Python 3.12 runners:

    <-- SNIP -->
    File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2164, in <module>
      register_finder(pkgutil.ImpImporter, find_on_path)
                        ^^^^^^^^^^^^^^^^^^^
    AttributeError: module 'pkgutil' has no attribute 'ImpImporter'. Did you mean: 'zipimporter'?
                    ^^^^^^^^^^^^^^^^^^^

This looks to be the issue[1] fixed in Pip 23.2 so use that verison

Link: pypa/pip#11501 [1]

.github/workflows/constraints.txt

@@ -1,2 +1,2 @@
-pip==22.3.1
+pip==23.2
 virtualenv==20.17.1