Bug 1638981 - Run 'mach reftest', 'mach crashtest', and 'mach jsrefte…

…st' with python 3; r=bc

These work for me now, thanks to your earlier work on reftest and my earlier
work on mochitest.

Differential Revision: https://phabricator.services.mozilla.com/D94892

mach

@@ -14,19 +14,16 @@
 py2commands="
     android
     awsy-test
-    crashtest
     firefox-ui-functional
     geckodriver
     geckodriver-test
     jsshell-bench
     marionette-test
-    jstestbrowser
     jstests
     mozharness
     prettier-format
     raptor
     raptor-test
-    reftest
     telemetry-tests-client
     test
     web-platform-tests