Bug 1620140 - Convert gen_scalar_enum.py to py3; r=firefox-build-system-reviewers,rstewart

Differential Revision: https://phabricator.services.mozilla.com/D65461

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

toolkit/components/telemetry/moz.build

@@ -168,7 +168,6 @@ GeneratedFile('TelemetryScalarData.h',
               inputs=scalar_files)
 GeneratedFile('TelemetryScalarEnums.h',
               script='build_scripts/gen_scalar_enum.py',
-              py2=True,
               inputs=scalar_files)
 
 # Generate the JSON scalar definitions. They will only be