Refs #265. Explicitly pin older packages compatible with these legacy pythons

Author: kylegibson-rldatix

.github/workflows/ci.yml

@@ -30,7 +30,7 @@ jobs:
           run: |
             cd /workspace
             python --version
-            pip install --requirement requirements/testing.txt
+            pip install Jinja2==2.10 MarkupSafe<2.0 coverage==3.6 nose==1.30 flake8==2.6.2
             pip install -e .
             nosetests --with-doctest