Small change to Schema model to allow generated specs to be picklable

- overriding __dict__ is death for pickling.   I renamed it to __newdict__
  and tweaked the one user of it, and now this part of the structure is
  working fine for pickling
- there are also upstream changes in jsonschema that need to be in for the
  overall success
- this allows me to create the API spec from a swagger file once (which takes
  2-20s for the files I'm working with), and cache the result as a pickle file
  for loading on the next startup (assuming the swagger file hasn't been
  updated).  The load from pickle files takes 2-5ms.  This is an improvement
  of load time by 3 orders of magnitude.

openapi_core/schema/schemas/models.py

@@ -67,8 +67,12 @@ class Schema(object):
 
         self._source = _source
 
+    # Overriding object.__dict__ is a VERY bad idea as it totally breaks any
+    # possibility of pickling this object.  Pickling marshalls via object.__dict__
+    # via default __getstate__ and __setstate__ methods.  This is now renamed to
+    # keep the functionality for the validators, but keep pickling operational.
     @property
-    def __dict__(self):
+    def __newdict__(self):
         return self._source or self.to_dict()
 
     def to_dict(self):

openapi_core/unmarshalling/schemas/factories.py

@@ -86,7 +86,7 @@ class SchemaUnmarshallersFactory(object):
         }
         if self.context is not None:
             kwargs[self.CONTEXT_VALIDATION[self.context]] = True
-        return OAS30Validator(schema.__dict__, **kwargs)
+        return OAS30Validator(schema.__newdict__, **kwargs)
 
     def _get_format_checker(self):
         fc = deepcopy(oas30_format_checker)