[1.7.x] Fixed flake8 warnings introduced in recent commits.

Backport of 79f05616fbf48cf7c205ef17666af0c3d47b3c1e from master

django/db/backends/postgresql_psycopg2/base.py

@@ -4,8 +4,6 @@ PostgreSQL database backend for Django.
 Requires psycopg 2: http://initd.org/projects/psycopg2
 """
 
-import sys
-
 from django.conf import settings
 from django.db.backends import (BaseDatabaseFeatures, BaseDatabaseWrapper,
     BaseDatabaseValidation)

tests/test_discovery_sample/doctests.py

@@ -3,6 +3,7 @@ Doctest example from the official Python documentation.
 https://docs.python.org/3/library/doctest.html
 """
 
+
 def factorial(n):
     """Return the factorial of n, an exact integer >= 0.
 

tests/test_runner/test_discover_runner.py

@@ -105,7 +105,6 @@ class DiscoverRunnerTest(TestCase):
 
     def test_testcase_ordering(self):
         suite = DiscoverRunner().build_suite(["test_discovery_sample/"])
-        tc_names = [case.__class__.__name__ for case in suite._tests]
         self.assertEqual(
             suite._tests[0].__class__.__name__,
             'TestDjangoTestCase',