Audformat

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* Fixed: reintroduce sorting the output of
``audformat.Database.files`` and
``audformat.Database.segments``

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* Fixed: changelog for 0.13.0

---------------------------

* Changed: ``audformat.utils.union()`` no longer sorts levels
* Changed: ``audformat.Table.save()`` forces pickle format 4
* Changed: clean up test requirements
* Changed: require ``pandas < 1.4.0``

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* Changed: the API documentation on the ``language`` argument
of ``audformat.Database`` is more verbose now
* Changed: the difference between
``audformat.define.DataType.TIME``
and ``audformat.define.DataType.DATE``
is now discussed in the API documentation
* Fixed: saving a not loaded table to CSV
when a PKL file is present
* Fixed: ``pandas`` deprecation warnings

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* Removed: Python 3.6 support

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* Added: ``audformat.assert_no_duplicates()``
* Changed: ``audformat.assert_index()`` no longer checks for duplicates