Let the mypy plugin manage extended-precision `numpy.number` subclasses
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The mypy_ plugin, introduced in `numpy/numpy#17843`_, has been expanded:
the plugin now removes annotations for platform-specific extended-precision
types that are not available to the platform in question.
For example, it will remove `~numpy.float128` when not available.

Without the plugin *all* extended-precision types will, as far as mypy is concerned,
be available on all platforms.

To enable the plugin, one must add it to their mypy `configuration file`_:

.. code-block:: ini

    [mypy]
    plugins = numpy.typing.mypy_plugin


.. _mypy: http://mypy-lang.org/
.. _configuration file: https://mypy.readthedocs.io/en/stable/config_file.html
.. _`numpy/numpy#17843`: https://github.com/numpy/numpy/pull/17843

Add a release note for the mypy plugin update
REL: Add a release note for the mypy plugin update
