style: format backend with black

Also drops the session-sliding comment in login_required.

backend/tests/test_passwords.py

@@ -14,7 +14,9 @@ KNOWN_ANSWER = "pbkdf2_sha256$1000$AAAAAAAAAAAAAAAAAAAAAA==$9ZLLEusnEPU3Km8h+vnd
 
 class PasswordHashTests(unittest.TestCase):
     def test_known_answer(self):
-        stored = hash_password("correct horse battery staple", PEPPER, salt=bytes(16), iterations=1000)
+        stored = hash_password(
+            "correct horse battery staple", PEPPER, salt=bytes(16), iterations=1000
+        )
         self.assertEqual(stored, KNOWN_ANSWER)
 
     def test_round_trip_uses_current_iterations_and_random_salt(self):
@@ -28,7 +30,9 @@ class PasswordHashTests(unittest.TestCase):
         self.assertFalse(verify_password("wrong password!!", KNOWN_ANSWER, PEPPER))
 
     def test_wrong_pepper_fails(self):
-        self.assertFalse(verify_password("correct horse battery staple", KNOWN_ANSWER, b"q" * 32))
+        self.assertFalse(
+            verify_password("correct horse battery staple", KNOWN_ANSWER, b"q" * 32)
+        )
 
     def test_needs_rehash_below_current_iterations(self):
         self.assertTrue(needs_rehash(KNOWN_ANSWER))