Bug 1617748 - Run `./mach build`, `./mach build-backend`, `./mach configure` with Python 3 r=dmajor

Differential Revision: https://phabricator.services.mozilla.com/D66107

--HG--
extra : moz-landing-system : lando

mach

@@ -17,14 +17,11 @@ py2commands="
     artifact
     awsy-test
     browsertime
-    build
-    build-backend
     buildsymbols
     cargo
     check-spidermonkey
     clang-format
     compileflags
-    configure
     cppunittest
     cramtest
     crashtest