Products.cmfcore

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- Avoid DeprecationWarning for changed import location for ``rfc1123_date``

- Fix several DeprecationWarnings during unit tests
(`112 <https://github.com/zopefoundation/Products.CMFCore/issues/112>`_)

- Set Cache-Control header in '304 Not Modified' response case as well.
(`111 <https://github.com/zopefoundation/Products.CMFCore/issues/111>`_)

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- Make sure ``getSkinNameFromRequest`` only returns sane values
(`109 <https://github.com/zopefoundation/Products.CMFCore/issues/109>`_)

- Fix Python 3 incompatibility in ``CookieCrumbler.credentialsChanged``

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- Update configuration for version 5 of ``isort``.

- Fix deprecation warnings occurring on Zope 5.

- Added support for Python 3.9.

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- Fix an issue with the indexing queue where an UNINDEX followed by REINDEX was
wrongly optimized to UNINDEX instead of REINDEX.
(`96 <https://github.com/zopefoundation/Products.CMFCore/pull/96>`_)

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- Change the default value of uid to None on CMFCatalogAware's reindexObject method (`87 <https://github.com/zopefoundation/Products.CMFCore/issues/87>`_)

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- Redid most filesystem content type ZMI Customize views.