Drop legacy python version support

[icemac]

Backwards incompatible changes:

• Drop support for Python 2.7, 3.5, 3.6.

• Drop modules which do not seem to be Python compatible:

  • zope.testing.loghandler
  • zope.testing.server

• Drop doctest option IGNORE_EXCEPTION_MODULE_IN_PYTHON2.

• Remove functions:

  • zope.testing.renormalizing.strip_dottedname_from_traceback
  • zope.testing.renormalizing.is_dotted_name

I semi randomly chose this repository to start ripping out Python 2.7 up to 3.6 support. This PR uses https://github.com/zopefoundation/meta/pull/178

[icemac]

Thank you for reviewing this PR. 😃