Bug 1681150 - python3 - testing/config/marionette_requirements.txt - exclude installing enum on python3 r=marionette-reviewers,jgraham

Differential Revision: https://phabricator.services.mozilla.com/D99027

testing/config/marionette_requirements.txt

@@ -2,7 +2,7 @@
 
 ../tools/wptserve
 ../tools/wpt_third_party/certifi
-../tools/wpt_third_party/enum
+../tools/wpt_third_party/enum ; python_version < '3'
 ../tools/wpt_third_party/h2
 ../tools/wpt_third_party/hyperframe
 ../tools/wpt_third_party/hpack