Backported and Experimental Type Hints for Python

 The "typing_extensions" module serves two related purposes:
 .
 1. Enable use of new type system features on older Python versions.
 2. Enable experimentation with new type system PEPs before they are accepted
    and added to the "typing" module.
 .
 "typing_extensions" is treated specially by static type checkers such as mypy
 and pyright. Objects defined in "typing_extensions" are treated the same
 way as equivalent forms in "typing".