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.

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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