[Checkins] [zopefoundation/RestrictedPython] c8f551: Fix test infrastructure:

[Michael Howitz]

  Branch: refs/heads/master
  Home:   https://github.com/zopefoundation/RestrictedPython
  Commit: c8f551d23a7bee341e90c94c51918b4c863eb96b
      https://github.com/zopefoundation/RestrictedPython/commit/c8f551d23a7bee341e90c94c51918b4c863eb96b
  Author: Michael Howitz <mh at gocept.com>
  Date:   2019-05-08 (Wed, 08 May 2019)

  Changed paths:
    M tox.ini

  Log Message:
  -----------
  Fix test infrastructure:

* Remove pytest plug-in which is not compatible with current pytest versions.
* Use Python 3 for the documentation to be able to use a current Sphinx version.