Instruct setuptools to tag wheels w/ py2.py3

This patch marks the wheels as compabible with Python 2 and Python 3
though a tag in the filename that dependency resolvers like the one in
pip would use to initially disregard candidates before looking into
their core packaging metadata.

Ref: pypa/setuptools#4939.

Author: webknjaz

setup.cfg

@@ -3,4 +3,13 @@ release = dists upload
 dists = clean --all sdist bdist_wheel
 
 [bdist_wheel]
-universal = 1
+# NOTE: This instructs `bdist_wheel` to create a wheel that with the
+# NOTE: tag "py2.py3" which implies that this wheel contains code
+# NOTE: compatible with both Python 2 and Python 3. Dependency resolvers
+# NOTE: will still look into what the "Requires-Python" field in the
+# NOTE: metadata has.
+# NOTE: This used to be configurable though the "universal = 1" setting
+# NOTE: but setuptools is deprecating it by the fall of 2025. Setting
+# NOTE: "python_tag" explicitly does exactly the same thing as
+# NOTE: "universal = 1" would.
+python_tag = py2.py3