CSTTransform.on_leave type definitions are too strict

The type definitions for `CSTTransform.on_leave` https://github.com/Instagram/LibCST/blob/df231f3fa5ac3dfa7b14a30103249e5da79509e3/libcst/_visitors.py#L50-L52 is too strict, as it requires that the return type is exactly the same as the original_node, which is not correct. For example, you could change a literal float to a predefined constant.