Remove optional dependencies (#4088)

* Stop using already_listening in standalone

* remove already_listening

* remove psutil entirely

* fix #595

* Add basic perform test

* make pep8 happy

* Add test_perform_eacces

* add _setup_perform_error

* Add test_perform_unexpected_socket_error

* add test_perform_eaddrinuse_no_retry

* add test_perform_eaddrinuse_retry

* cleanup tests

* stop using dnspython

* don't install dns extras in tox

* remove dns extras from setup.py

* Add simple_verify back to DNS response

* remove dnspython from oldest tests

setup.py

@@ -67,7 +67,6 @@ dev_extras = [
     'astroid==1.3.5',
     'coverage',
     'nose',
-    'psutil>=2.2.1',  # for tests, optional
     'pylint==1.4.2',  # upstream #248
     'tox',
     'twine',