Fix deprecation warnings

There is a known deprecation issue and we don't need to be bothered by
it for now.  Eventually upstream will fix it in which case we should
remove the line that silences the warning.

tests/test_presigned_posts.py

@@ -38,6 +38,9 @@ def test_create_presigned_post_returns_boto3_result(mocker):
     }
 
 
+@pytest.mark.filterwarnings(
+    "ignore:datetime.datetime.utcnow"
+)  # See https://github.com/boto/boto3/issues/3889
 @pytest.mark.integration
 def test_create_presigned_post_properly_intgrates_with_boto3():
     response = client.post(