debian/changelog

+python-pypubsub (4.0.3-4) unstable; urgency=medium
+
+  * Breaks+Replaces: python3-pypubsub
+    Closes: #946425
+
+ -- Andreas Tille <tille@debian.org>  Mon, 09 Dec 2019 08:38:32 +0100
+
 python-pypubsub (4.0.3-3) unstable; urgency=medium
 
   * Rename python3-pbpubsub to python3-pubsub to follow DPMT policy

debian/control

@@ -17,6 +17,9 @@ Package: python3-pubsub
 Architecture: all
 Depends: ${python3:Depends},
          ${misc:Depends}
+Breaks: python3-pypubsub
+Provides: python3-pypubsub
+Replaces: python3-pypubsub
 Description: Python 3 publish-subcribe library
  Provides a publish-subscribe API to facilitate event-based or message-based
  architecture in a single-process application. It is pure Python