Bug 1621452 - Generate xpidllex.py and xpidlyacc.py with python2. r=rstewart

Ply doesn't actually support python3. The generation of the files seems
to work with python3 for some reason, but running the corresponding
tests doesn't. The only version of ply that supports python3 is
currently in beta, so upgrading ply is probably not an option at the
moment.

Other commands that do use ply (mach webidl-parser-test and
ipc/ipdl/Makefile.in) have not switched to python3 (probably because
they don't work). Let's go with the simpler thing for now: switch back
to python 2 for xpidllex.py and xpidlyacc.py, which will make them
deterministically created again.

Differential Revision: https://phabricator.services.mozilla.com/D67400

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

xpcom/idl-parser/xpidl/moz.build

@@ -9,4 +9,4 @@ PYTHON_UNITTEST_MANIFESTS += [
 ]
 
 GeneratedFile('xpidl.stub', 'xpidllex.py', 'xpidlyacc.py', script='header.py',
-              entry_point='main')
+              entry_point='main', py2=True)