Mail related fixtures, bug fixes; requires pytest 3.6

Features
^^^^^^^^

• 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).

Support Django 2.0, features, fixes

3.2.0

Features
^^^^^^^^

• Added new fixture django_assert_num_queries for testing the number of
  database queries (#387).
• --fail-on-template-vars has been improved and should now return
  full/absolute path (#470).
• Support for setting the live server port (#500).
• unittest: help with setUpClass not being a classmethod (#544).

Bug fixes
^^^^^^^^^

• Fix --reuse-db and --create-db not working together (#411).
• Numerous fixes in the documentation. These should not go unnoticed 🌟

Compatibility
^^^^^^^^^^^^^

• Support for Django 2.0 has been added.
• Support for Django before 1.8 has been dropped.