Fixes macOS build on Python 3 (#563) The error was: ``` Traceback (most recent call last): File "_generate-darwin-source-and-headers.py", line 209, in <module> generate_source_and_headers(generate_osx=not args.only_ios, generate_ios=not args.only_osx) File "_generate-darwin-source-and-headers.py", line 197, in generate_source_and_headers for header_name, tag_tuples in platform_headers.iteritems(): AttributeError: 'collections.defaultdict' object has no attribute 'iteritems' ```