Provide Rust wrappers around PyFrozenDict

frozendict has been introduced in Python 3.15 and currently pyo3 only expose PyFrozenDict_New through the ffi module.

I believe it would be interesting to provide at PyFrozenDict struct and a PyFrozenDictMethods trait to allow high level manipulation of object of this type from Rust.

Additionally check methods (PyAnyDict_Check and PyAnyDict_CheckExact) should be exposed in the ffi, like the set counterparts.

I can try to work on that but I would be happy to have some pointers about the best way to attack this.