This was originally a backport of the standard library 'typing' module to
Python versions older than 3.5.

Typing defines a standard notation for Python function and variable type
annotations. The notation can be used for documenting code in a concise,
standard format, and it has been designed to also be used by static and runtime
type checkers, static analyzers, IDEs and other tools.

Today, 'typing_extensions' serves two related purposes:

- Enable use of new type system features on older Python versions. For example,
  'typing.TypeGuard' is new in Python 3.10, but 'typing_extensions' allows
  users on Python 3.6 through 3.9 to use it too.
- Enable experimentation with new type system PEPs before they are accepted and
  added to the 'typing' module.