Use pytest instead of py.test

py.test is the old way of running pytest.

Also remove DJANGO_SETTINGS_MODULE env when running tests

This is already defined in pytest.ini and py.test will take it from
as it should also take parameters we might add in pytest.ini in the future.

Author: Oliver Sauder

README.rst

@@ -142,7 +142,7 @@ installed and activated:
 
     $ pip install -r requirements-development.txt
     $ flake8
-    $ DJANGO_SETTINGS_MODULE=example.settings.test py.test
+    $ pytest
 
 -----
 Usage