Bug 1638950 - run mach browsertime with python3 r=sparky

Changes:
  - `mach browsertime` appears to run well on python3 without any changes.

Differential Revision: https://phabricator.services.mozilla.com/D81383

mach

@@ -13,7 +13,6 @@
 py2commands="
     android
     awsy-test
-    browsertime
     check-spidermonkey
     clang-format
     cramtest