bpo-28556: Update the opening note in typing docs (GH-16204)

This PR replaces the old note mentioning that `typing` is a provisional module with a new one mentioning types are not enforced at runtime. I am not sure if there was any official announcement about making `typing` non-provisional, but _de-facto_ no new features were added during Python 3.7, and no backwards incompatible changes were made except for few small things that were considered bugs.
(cherry picked from commit 81528ba)

Co-authored-by: Ivan Levkivskyi <[email protected]>

Author: miss-islington

Doc/library/typing.rst

@@ -10,10 +10,9 @@
 
 .. note::
 
-   The typing module has been included in the standard library on a
-   :term:`provisional basis <provisional api>`. New features might
-   be added and API may change even between minor releases if deemed
-   necessary by the core developers.
+   The Python runtime does not enforce function and variable type annotations.
+   They can be used by third party tools such as type checkers, IDEs, linters,
+   etc.
 
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