Bug 1550956: Import builtins in Python 3 compatible way r=firefox-build-system-reviewers,chmanchester

Differential Revision: https://phabricator.services.mozilla.com/D30790

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

build/mach_bootstrap.py

@@ -11,7 +11,10 @@ import platform
 import subprocess
 import sys
 import uuid
-import __builtin__
+if sys.version_info[0] < 3:
+    import __builtin__ as builtins
+else:
+    import builtins
 
 from types import ModuleType
 
@@ -434,4 +437,4 @@ class ImportHook(object):
 
 
 # Install our hook
-__builtin__.__import__ = ImportHook(__builtin__.__import__)
+builtins.__import__ = ImportHook(builtins.__import__)