diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 9248eb2..24b92eb 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -67,7 +67,7 @@
 _DEFAULT_TOKEN_LIFETIME_SECS = 3600  # 1 hour in seconds
 _DEFAULT_MAX_CACHE_SIZE = 10
 _ALGORITHM_TO_VERIFIER_CLASS = {"RS256": crypt.RSAVerifier}
-_CRYPTOGRAPHY_BASED_ALGORITHMS = set(["ES256"])
+_CRYPTOGRAPHY_BASED_ALGORITHMS = frozenset(["ES256"])
 
 if es256 is not None:  # pragma: NO COVER
     _ALGORITHM_TO_VERIFIER_CLASS["ES256"] = es256.ES256Verifier
diff --git a/tests/transport/test_requests.py b/tests/transport/test_requests.py
index d6770de..d24b6f6 100644
--- a/tests/transport/test_requests.py
+++ b/tests/transport/test_requests.py
@@ -55,12 +55,12 @@
 
     def test_tracks_elapsed_time_w_numeric_timeout(self, frozen_time):
         with self.make_guard(timeout=10) as guard:
-            frozen_time.tick(delta=3.8)
+            frozen_time.tick(delta=datetime.timedelta(seconds=3.8))
         assert guard.remaining_timeout == 6.2
 
     def test_tracks_elapsed_time_w_tuple_timeout(self, frozen_time):
         with self.make_guard(timeout=(16, 19)) as guard:
-            frozen_time.tick(delta=3.8)
+            frozen_time.tick(delta=datetime.timedelta(seconds=3.8))
         assert guard.remaining_timeout == (12.2, 15.2)
 
     def test_noop_if_no_timeout(self, frozen_time):
@@ -72,13 +72,13 @@
     def test_timeout_error_w_numeric_timeout(self, frozen_time):
         with pytest.raises(requests.exceptions.Timeout):
             with self.make_guard(timeout=10) as guard:
-                frozen_time.tick(delta=10.001)
+                frozen_time.tick(delta=datetime.timedelta(seconds=10.001))
         assert guard.remaining_timeout == pytest.approx(-0.001)
 
     def test_timeout_error_w_tuple_timeout(self, frozen_time):
         with pytest.raises(requests.exceptions.Timeout):
             with self.make_guard(timeout=(11, 10)) as guard:
-                frozen_time.tick(delta=10.001)
+                frozen_time.tick(delta=datetime.timedelta(seconds=10.001))
         assert guard.remaining_timeout == pytest.approx((0.999, -0.001))
 
     def test_custom_timeout_error_type(self, frozen_time):
@@ -87,7 +87,7 @@
 
         with pytest.raises(FooError):
             with self.make_guard(timeout=1, timeout_error_type=FooError):
-                frozen_time.tick(2)
+                frozen_time.tick(delta=datetime.timedelta(seconds=2))
 
     def test_lets_suite_errors_bubble_up(self, frozen_time):
         with pytest.raises(IndexError):
@@ -222,7 +222,7 @@
             authed_session.request("GET", self.TEST_URL)
 
         expected_timeout = google.auth.transport.requests._DEFAULT_TIMEOUT
-        assert patched_request.call_args.kwargs.get("timeout") == expected_timeout
+        assert patched_request.call_args[1]["timeout"] == expected_timeout
 
     def test_request_no_refresh(self):
         credentials = mock.Mock(wraps=CredentialsStub())