Documenttemplate

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- Set the ``tree-s`` cookie for ``dtml-tree`` tags with ``SameSite=Lax``.
The tree tag never set this attribute. That causes modern browsers to show
warnings in the browser console and break tree displays in the future.
See https://hacks.mozilla.org/2020/08/changes-to-samesite-cookie-behavior/
for information about the ``SameSite`` cookie attribute and why its handling
in browsers is changing.

- Add support for Python 3.10.

- Drop support for Python 3.5.

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- Make ``ustr.ustr`` Python 3 compatible
(`Zope921 <https://github.com/zopefoundation/Zope/issues/921>`_)

- Add support for Python 3.9

- Restore ``sql_quote`` behavior of always returning native strings
(`54 <https://github.com/zopefoundation/DocumentTemplate/issues/54>`_)

- Fix broken tree tag
(`52 <https://github.com/zopefoundation/DocumentTemplate/issues/52>`_)

- Drop support for Python 2.

- Eventually drop BBB code leading to a deprecation warning in version 3.2+.
(`42 <https://github.com/zopefoundation/DocumentTemplate/issues/42>`_)

- Update `isort` to version 5.

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- de-fang ``sql_quote`` even more as quoting is too database-specific.
(`48 <https://github.com/zopefoundation/DocumentTemplate/issues/48>`_)

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- prevent a really strange ``AccessControl`` test failure when running
Zope's ``alltests`` script by importing deprecated names from
``zope.sequencesort.ssort`` instead of ``sequence/SortEx.py`` in
``sequence/__init__.py``

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- no longer escape double quotes in ``sql_quote`` - that breaks PostgreSQL
(`48 <https://github.com/zopefoundation/DocumentTemplate/issues/48>`_)

- Added `DeprecationWarnings` for all deprecated files and names
(`42 <https://github.com/zopefoundation/DocumentTemplate/issues/42>`_)

- Import sorting done like Zope itself

- Applied extended linting configuration similar to Zope's own

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- Escape more characters in ``sql_quote``. Taken over from PloneHotfix20200121.