Use a different warning class so users get warnings (#4014)

* Use a different warning class so users get warnings

* fixed tests

* do our own warning class

* typo

* flake8
diff --git a/tests/x509/test_x509.py b/tests/x509/test_x509.py
index 97b5a74..27e284a 100644
--- a/tests/x509/test_x509.py
+++ b/tests/x509/test_x509.py
@@ -577,7 +577,7 @@
             backend
         )
 
-        with pytest.deprecated_call():
+        with pytest.warns(utils.CryptographyDeprecationWarning):
             assert cert.serial == 2
             assert cert.serial_number == 2
 
@@ -588,7 +588,7 @@
             backend
         )
 
-        with pytest.deprecated_call():
+        with pytest.warns(utils.CryptographyDeprecationWarning):
             cert.serial
 
     def test_load_der_cert(self, backend):