Another attempt at fixing e2e tests (openedx-unsupported#2931)

Pin pip to 20.0.2

Author: mulby

Makefile

@@ -43,6 +43,7 @@ requirements: requirements.js
 	pip install -r requirements/dev.txt --exists-action w
 
 requirements.tox:
+	pip install -U pip==20.0.2
 	pip install -r requirements/tox.txt --exists-action w
 
 production-requirements: requirements.js