Add Python 2.3 support

This commit also adds the 'six' dependency

vcr/serializers/yamlserializer.py

@@ -1,4 +1,6 @@
+import sys
 import yaml
+from . import compat
 
 # Use the libYAML versions if possible
 try:
@@ -6,11 +8,38 @@ try:
 except ImportError:
     from yaml import Loader, Dumper
 
+"""
+Just a general note on the serialization philosophy here:
+I prefer cassettes to be human-readable if possible.  Yaml serializes
+bytestrings to !!binary, which isn't readable, so I would like to serialize to
+strings and from strings, which yaml will encode as utf-8 automatically.
+All the internal HTTP stuff expects bytestrings, so this whole serialization
+process feels backwards.
+
+Serializing: bytestring -> string (yaml persists to utf-8)
+Deserializing: string (yaml converts from utf-8) -> bytestring
+"""
+
+def _restore_frozenset():
+    """
+    Restore __builtin__.frozenset for cassettes serialized in python2 but 
+    deserialized in python3 and builtins.frozenset for cassettes serialized
+    in python3 and deserialized in python2
+    """
+
+    if '__builtin__' not in sys.modules:
+        import builtins
+        sys.modules['__builtin__'] = builtins
+
+    if 'builtins' not in sys.modules:
+        sys.modules['builtins'] = sys.modules['__builtin__']
 
 def deserialize(cassette_string):
+    _restore_frozenset()
     data = yaml.load(cassette_string, Loader=Loader)
     requests = [r['request'] for r in data]
     responses = [r['response'] for r in data]
+    responses = [compat.convert_to_bytes(r['response']) for r in data]
     return requests, responses
 
 
@@ -20,6 +49,6 @@ def serialize(cassette_dict):
         'response': response,
     } for request, response in zip(
         cassette_dict['requests'],
-        cassette_dict['responses']
+        [compat.convert_to_unicode(r) for r in cassette_dict['responses']],
     )])
     return yaml.dump(data, Dumper=Dumper)