Drop Python 2.7 and 3.4 support

* Update setup.py requires and classifiers
* Drop Python 2.7 and 3.4 from CI
* Update docs dropping 2.7 and 3.4 support
* Fix mock imports and remove tests related to pypi's mock module
* Add py27 and 34 support docs to the sidebar
* Remove usage of six from tmpdir
* Remove six.PY* code blocks
* Remove sys.version_info related code
* Cleanup compat
* Remove obsolete safe_str
* Remove obsolete __unicode__ methods
* Remove compat.PY35 and compat.PY36: not really needed anymore
* Remove unused UNICODE_TYPES
* Remove Jython specific code
* Remove some Python 2 references from docs

Related to #5275

testing/example_scripts/acceptance/fixture_mock_integration.py

@@ -1,10 +1,6 @@
 # -*- coding: utf-8 -*-
 """Reproduces issue #3774"""
-
-try:
-    import mock
-except ImportError:
-    import unittest.mock as mock
+from unittest import mock
 
 import pytest