Mail related fixtures, bug fixes; requires pytest 3.6

Features
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• Added new fixtures django_mail_dnsname and django_mail_patch_dns,
  used by mailoutbox to monkeypatch the DNS_NAME used in
  :py:mod:django.core.mail to improve performance and
  reproducibility.

Bug fixes
^^^^^^^^^

• Fixed test for classmethod with Django TestCases (#597, #598).
• Fixed RemovedInPytest4Warning: MarkInfo objects are deprecated (#596, #603)
• Fixed scope of overridden settings with live_server fixture: previously they
  were visible to following tests (#612).