lib/python/types.* : Add type hints, docstrings,

Summary:
The main motivation here is to improve overall code quality and maintainability, through documentation, typing and method naming.

The addition of type hints in Cython code also affects type safety, as previously unchecked calls will now be checked against the declared types (and raise `TypeError` on mismatch).

Reviewed By: yoney

Differential Revision: D57443704

fbshipit-source-id: 1f4f078041655261d21440bff0f68cea21c9dae6

third-party/thrift/src/thrift/lib/python/types.pxd

@@ -216,7 +216,7 @@ cdef class Union(StructOrUnion):
     cdef readonly object value
     cdef void _fbthrift_load_cache(self) except *
     cdef object _fbthrift_to_internal_data(self, type_value, value)
-    cdef void _fbthrift_update_type_value(self, type_value, value) except *
+    cdef void _fbthrift_set_union_value(self, int field_id, object value) except *
     cdef folly.iobuf.IOBuf _serialize(Union self, Protocol proto)
     cdef uint32_t _deserialize(Union self, folly.iobuf.IOBuf buf, Protocol proto) except? 0