Remove API deprecated in 1.6, clean up the legacy deprecations (#3468)

* Remove API deprecated in 1.6, clean up the legacy deprecations

* flake8, unused import
diff --git a/tests/test_x509.py b/tests/test_x509.py
index db26f56..de19d0d 100644
--- a/tests/test_x509.py
+++ b/tests/test_x509.py
@@ -521,7 +521,7 @@
         )
 
         with warnings.catch_warnings():
-            warnings.simplefilter("always", utils.DeprecatedIn10)
+            warnings.simplefilter("always", utils.PersistentlyDeprecated)
             assert cert.serial == 2
             assert cert.serial_number == 2
 
@@ -533,7 +533,7 @@
         )
 
         with warnings.catch_warnings():
-            warnings.simplefilter("always", utils.DeprecatedIn10)
+            warnings.simplefilter("always", utils.PersistentlyDeprecated)
             with pytest.deprecated_call():
                 cert.serial