test_auth_backends: Remove now-unnecessary compatibility code.

This code was added as part of the Django 1.10 migration to make our
tests work with both Django 1.8 and 1.10.  Now that we're on 1.10,
it's no longer required.

tools/test-backend

@@ -82,7 +82,6 @@ not_yet_fully_covered = {
     'zerver/models.py',
     # Test files should have full coverage; it's a bug in the test if
     # they don't!
-    'zerver/tests/test_auth_backends.py',
     'zerver/tests/test_narrow.py',
     'zerver/tests/test_tornado.py',
     'zerver/tests/test_urls.py',

zerver/tests/test_auth_backends.py

@@ -1014,24 +1014,16 @@ class TestDevAuthBackend(ZulipTestCase):
         data = {'direct_email': email}
         with self.settings(AUTHENTICATION_BACKENDS=('zproject.backends.EmailAuthBackend',)):
             with self.assertRaisesRegex(Exception, 'Direct login not supported.'):
-                try:
                 with mock.patch('django.core.handlers.exception.logger'):
                     self.client_post('/accounts/login/local/', data)
-                except ImportError:
-                    with mock.patch('django.core.handlers.base.logger'):
-                        self.client_post('/accounts/login/local/', data)
 
     def test_login_failure_due_to_nonexistent_user(self):
         # type: () -> None
         email = 'nonexisting@zulip.com'
         data = {'direct_email': email}
         with self.assertRaisesRegex(Exception, 'User cannot login'):
-            try:
             with mock.patch('django.core.handlers.exception.logger'):
                 self.client_post('/accounts/login/local/', data)
-            except ImportError:
-                with mock.patch('django.core.handlers.base.logger'):
-                    self.client_post('/accounts/login/local/', data)
 
 class TestZulipRemoteUserBackend(ZulipTestCase):
     def test_login_success(self):