Generate and use pytest XML reports

This generates XML reports when we run pytest through poe, and
uploads them in the relevant CI stage.

I've also ignored mypy cache files - I'm not sure why this hasn't
been an issue before...

.gitlab-ci.yml

@@ -102,6 +102,11 @@ pytest:
     - tags
     - web
 
+  artifacts:
+    when: always
+    reports:
+      junit: pytest_report.xml
+
 openapi:
   stage: testing
   image: python:3.7