[Checkins] [zopefoundation/Zope] eeb675: Restore "attrs" template variable support (#861)

[Jens Vagelpohl]

  Branch: refs/heads/master
  Home:   https://github.com/zopefoundation/Zope
  Commit: eeb675c42f5e77a77e512745ba299bc42b6d5af4
      https://github.com/zopefoundation/Zope/commit/eeb675c42f5e77a77e512745ba299bc42b6d5af4
  Author: Jens Vagelpohl <jens at netz.ooo>
  Date:   2020-07-06 (Mon, 06 Jul 2020)

  Changed paths:
    M CHANGES.rst
    M docs/zopebook/AppendixC.rst
    M src/Products/PageTemplates/engine.py
    M src/Products/PageTemplates/tests/testC2ZContext.py
    A src/Products/PageTemplates/tests/testVariables.py

  Log Message:
  -----------
  Restore "attrs" template variable support (#861)

* fix lost "attrs" variable

* provide `CHANGES.rst` entries

* fix `isort` problem

* fix typo

* tentatively remove workarounds for `https://github.com/malthe/chameleon/issues/323`

* Revert "tentatively remove workarounds for `https://github.com/malthe/chameleon/issues/323`"

This reverts commit 5f492756bbf97a1514a4e47c62709d949bc96ba1.

At least for the moment, `chameleon` is not yet ready to drop the work around.
In addition, our test suite is not sufficiently complete to verify the safety
of all `chameleon` features without the work around in place.

* clearly state in the comments related to `chameleon` work arounds that those might go away with ``chameleon > 3.8.0``;
add a not to the Zope Book that ``chameleon`` implements `attrs` and `default` in a special way and that their value should not be changed

* add test for `CONTEXTS`; restore `CONTEXTS` related documentation (improved)

Co-authored-by: Jens Vagelpohl <jens at netz.ooo>