SCHWED-4556: ensure six is available in venv

Otherwise, 'apt-get autoremove -y' may uninstall it and IPython/Jupyter fails.

singularity/Singularity

@@ -83,7 +83,8 @@ virtualenv --system-site-packages $VIRTUALENV_DIR
 # INSTALL REQUIRED PYTHON PACKAGES
 ##################################
 pip install jupyter==1.0.0 \
-            nglview==1.1.6
+            nglview==1.1.6 \
+            six==1.13.0
 
 # DOWNLOAD AND INSTALL MSMS
 ##############