pin edx-when to 2.1.0 for testing

requirements/constraints.txt

-# Version constraints for pip-installation.
+https://imgur.com/gallery/rjiMxVD# Version constraints for pip-installation.
 #
 # This file doesn't install any packages. It specifies version constraints
 # that will be applied if a package is needed.
@@ -109,3 +109,7 @@ pylint<2.10.0
 # Feel free to loosen this constraint if/when it is confirmed that a later
 # version of py2neo will work with Neo4j 3.5.
 py2neo<2022
+
+# as of right now (Sep 8 2021), we need to not use the latest edx-when
+# version which might be causing issues with course publishing
+edx-when==2.1.0