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'
```

diff --git a/generate-darwin-source-and-headers.py b/generate-darwin-source-and-headers.py
index b8c28e643fe896a4f7cb1557799d5bff0d150f32..516464f524f52f43344085a47f910a42e575b47c 100755 (executable)
--- a/generate-darwin-source-and-headers.py
+++ b/generate-darwin-source-and-headers.py
@@ -186,7 +186,7 @@ def generate_source_and_headers(generate_osx=True, generate_ios=True):
         build_target(desktop64_platform, platform_headers)
 
     mkdir_p('darwin_common/include')
-    for header_name, tag_tuples in platform_headers.iteritems():
+    for header_name, tag_tuples in platform_headers.items():
         basename, suffix = os.path.splitext(header_name)
         with open(os.path.join('darwin_common/include', header_name), 'w') as header:
             for tag_tuple in tag_tuples: