refactor: latest Django 2.2.21 breaks the tests.

Its mostly due the more restriction on empty file names and paths with dot segments during uploads

requirements/constraints.txt

@@ -12,6 +12,9 @@
 # This file contains all common constraints for edx-repos
 -c https://raw.githubusercontent.com/edx/edx-lint/master/edx_lint/files/common_constraints.txt
 
+# latest version breaks the tests.
+django<2.2.21
+
 # celery 5.0 has dropped python3.5 support.
 celery<5.0