Use the py2+py3 compatible replacement for execfile(...)

Author: practicalswift

test/Driver/Dependencies/Inputs/update-dependencies-bad.py

@@ -45,4 +45,4 @@
         sys.exit(129)
 
 execDir = os.path.dirname(os.path.abspath(__file__))
-execfile(os.path.join(execDir, "update-dependencies.py"))
+exec(open(os.path.join(execDir, "update-dependencies.py")).read())