Mk/Uses: Add new variable PY_TYPING_EXTENSIONS

devel/py-typing-extensions is a common dependency while the python
version is less than 3.11. To make things easier for committers add a
new var PY_TYPING_EXTENSIONS only when the python version is less than
3.11. This variable is not supposed to be used when typing-extensions is
a hard requirement irrespective of python versions.