Add pip conflict checks to testing pipeline

tox.ini

@@ -5,6 +5,7 @@ skipsdist = True
 [testenv]
 changedir={toxinidir}/website
 commands =
+    pipconflictchecker  # if this fails, use pipdeptree to debug
     python manage.py check
     python manage.py templatecheck --project-only
     python manage.py makemigrations --no-input --check --dry-run
@@ -12,6 +13,7 @@ commands =
 deps = 
     -r{toxinidir}/requirements.txt
     coverage
+    pip-conflict-checker
     {env:CI_DEPS:}
 
 passenv = GITLAB_CI