Backward compatible Update to support Python 3.10 that Passes all unit tests

[warnes]

This pull makes three minor changes to enable pyswagger to be used with Python 3.10 and the current versions of PyYaml and Flask.

These changes should be backward compatible, and pass all unit tests.

1. Adapt to the move of collections.Mapping and collections.MutableMapping to collections.abc.Mapping and collections.abc.MutableMapping.
2. Use PyYAML's yaml.safe_load instead of yaml.load, which now requires an additional argument, and also prevent arbitrary code execution.
3. Update the flask tests to account for a change in how multiple values for the same header key are handled.