Update mailing list link in glossary

Per https://groups.google.com/d/msg/pypa-dev/rUNsfIbruHM/LCEx-CB5AgAJ
the pypa-dev Google Group is now decommissioned.
Pointing to distutils-sig and Discourse instead.

Signed-off-by: Sumana Harihareswara <sh@changeset.nyc>

Author: brainwane

source/glossary.rst

@@ -128,11 +128,14 @@ Glossary
 
     Python Packaging Authority (PyPA)
 
-        PyPA is a working group that maintains many of the relevant projects in
-        Python packaging. They maintain a site at https://www.pypa.io, host projects
-        on `github <https://github.com/pypa>`_ and `bitbucket
-        <https://bitbucket.org/pypa>`_, and discuss issues on the `pypa-dev
-        mailing list <https://groups.google.com/forum/#!forum/pypa-dev>`_.
+        PyPA is a working group that maintains many of the relevant
+        projects in Python packaging. They maintain a site at
+        https://www.pypa.io, host projects on `GitHub
+        <https://github.com/pypa>`_ and `Bitbucket
+        <https://bitbucket.org/pypa>`_, and discuss issues on the
+        `distutils-sig mailing list
+        <https://mail.python.org/mailman3/lists/distutils-sig.python.org/>`_
+	and `the Python Discourse forum <https://discuss.python.org/c/packaging>`__.
 
 
     Python Package Index (PyPI)