add constraint for edx-drf-extensions

There is a planned edx-drf-extension update to 3.0.0 to remove the
toggle used for ENFORCE_JWT_SCOPES. Currently, edx-platform is dependent
on the this toggle. This constraint will keep other `make upgrade` PRs
working in the short window before the edx-paltform PR to remove the
same toggle can land.

BOM-1324

requirements/constraints.txt

@@ -82,3 +82,6 @@ django-storages<1.9
 
 # Support was dropped for Python 3.5, Django 2.0, Django 2.1, DRF 3.7
 drf-yasg<1.17.1
+
+# BOM-1324: This constraint will be removed separately once 3.0.0 is out
+edx-drf-extensions<3.0.0