docs(install): document how to install gpg 2.0

Add link in install doc to upgrading doc and document pip command there.

Author: rouilj

doc/installation.txt

@@ -248,7 +248,11 @@ gpg
   If gpg_ is installed you can configure the mail gateway to perform
   verification or decryption of incoming OpenPGP MIME messages. When
   configured, you can require email to be cryptographically signed
-  before roundup will allow it to make modifications to issues.
+  before roundup will allow it to make modifications to issues. (Note
+  with python 3.13 or newer on some platforms, you may need to install
+  version 2.0 of gpg from test.pypi.org. See the `gpg install
+  directions in the upgrading document`_.
+
 
 jinja2
   To use the jinja2 template (may still be experimental, check out
@@ -318,6 +322,9 @@ requests
 .. _Using Redis for Session Databases:
    admin_guide.html#using-redis-for-session-databases
 
+.. _gpg install directions in the upgrading document:
+   upgrading.html#directions-for-installing-gpg
+
 Installing Roundup
 ==================
 

doc/upgrading.txt

@@ -441,6 +441,23 @@ the database (client.db), form (client.form).
 
 You can find an example in :ref:`dynamic_csp`.
 
+Directions for installing gpg
+-----------------------------
+
+In this release a new version of the gpg module was needed for Ubuntu
+24.04 and python 3.13. Paul Schwabauer produced a new version of the
+gpg module. However it is only on the test instance of pypi. If you
+run into issues installing gpg with pip, you can use::
+
+    pip install --index-url https://test.pypi.org/simple/ \
+                --extra-index-url https://pypi.org/simple gpg;
+
+to installed version 2.0 of gpg from test.pypi.org obtaining it's
+requirements from pypi.org.
+
+When `issue2551368 <https://issues.roundup-tracker.org/issue2551368>`_
+is closed, you should be able to use ``pip install gpg`` again.   
+
 .. index:: Upgrading; 2.3.0 to 2.4.0
 
 Migrating from 2.3.0 to 2.4.0