fix weakref in dockerfile

ci/docker_images/ubuntu/disco/Dockerfile

@@ -243,7 +243,10 @@ RUN cd /tmp \
   && pip install . \
   && cd - \
   && rm -Rf python-flint
- 
+
+# fix backports.weakref
+RUN pip uninstall -y backports.weakref
+RUN apt-get install --reinstall -y python-backports.weakref
 
 # ensure all libraries are known by the runtime linker
 RUN ldconfig