Add changelog for v1.3.1

docs/whats_new.rst

@@ -11,6 +11,25 @@ Changelog
 This page keeps a detailed human friendly rendering of what's new and changed
 in specific versions.
 
+.. _vp1p3p1:
+
+v1.3.1
+--------
+
+Minor bug fix release.
+
+Bug Fixes
+~~~~~~~~~~~
+
+- Fix fetching invites in guilds that the user is not in.
+- Fix the channel returned from :meth:`Client.fetch_channel` raising when sending messages. (:issue:`2531`)
+
+Miscellaneous
+~~~~~~~~~~~~~~
+
+- Fix compatibility warnings when using the Python 3.9 alpha.
+- Change the unknown event logging from WARNING to DEBUG to reduce noise.
+
 .. _vp1p3p0:
 
 v1.3.0