Bug 1677775 - enable Black on .mozbuild files r=linter-reviewers,firefox-build-system-reviewers,ahal,glandium

Differential Revision: https://phabricator.services.mozilla.com/D193005

tools/lint/black.yml

@@ -10,6 +10,7 @@ black:
     extensions:
         - build
         - configure
+        - mozbuild
         - py
     support-files:
         - 'tools/lint/python/**'