Bug 1678663 - Only install enum for python 2, r=karlcow

It's not the same as the stdlib for 3.6+ and is present on 3

Differential Revision: https://phabricator.services.mozilla.com/D99244

testing/config/marionette_source_requirements.txt

@@ -4,7 +4,7 @@
 
 ../web-platform/tests/tools/wptserve
 ../web-platform/tests/tools/third_party/certifi
-../web-platform/tests/tools/third_party/enum
+../web-platform/tests/tools/third_party/enum ; python_version < '3'
 ../web-platform/tests/tools/third_party/h2
 ../web-platform/tests/tools/third_party/hyperframe
 ../web-platform/tests/tools/third_party/hpack