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Sync what's new in 3.9 with 3.9 branch (GH-23943)
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Doc/whatsnew/3.9.rst

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@@ -1482,4 +1482,37 @@ and to match the behavior of static type checkers specified in the PEP.
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File "<stdin>", line 1, in <module>
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TypeError: unhashable type: 'set'
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(Contributed by Yurii Karabas in :issue:`42345`.)
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(Contributed by Yurii Karabas in :issue:`42345`.)
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macOS 11.0 (Big Sur) and Apple Silicon Mac support
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--------------------------------------------------
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As of 3.9.1, Python now fully supports building and running on macOS 11.0
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(Big Sur) and on Apple Silicon Macs (based on the ``ARM64`` architecture).
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A new universal build variant, ``universal2``, is now available to natively
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support both ``ARM64`` and ``Intel 64`` in one set of executables. Binaries
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can also now be built on current versions of macOS to be deployed on a range
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of older macOS versions (tested to 10.9) while making some newer OS
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functions and options conditionally available based on the operating system
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version in use at runtime ("weaklinking").
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(Contributed by Ronald Oussoren and Lawrence D'Anna in :issue:`41100`.)
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Notable changes in Python 3.9.2
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===============================
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collections.abc
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---------------
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:class:`collections.abc.Callable` generic now flattens type parameters, similar
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to what :data:`typing.Callable` currently does. This means that
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``collections.abc.Callable[[int, str], str]`` will have ``__args__`` of
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``(int, str, str)``; previously this was ``([int, str], str)``. To allow this
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change, :class:`types.GenericAlias` can now be subclassed, and a subclass will
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be returned when subscripting the :class:`collections.abc.Callable` type.
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Code which accesses the arguments via :func:`typing.get_args` or ``__args__``
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need to account for this change. A :exc:`DeprecationWarning` may be emitted for
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invalid forms of parameterizing :class:`collections.abc.Callable` which may have
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passed silently in Python 3.9.1. This :exc:`DeprecationWarning` will
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become a :exc:`TypeError` in Python 3.10.
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(Contributed by Ken Jin in :issue:`42195`.)

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