Don't raise an UnsupportedExtension for critical extensions. (#3550)

* Don't raise an UnsupportedExtension for critical extensions.

Fixes #2903
Fixes #2901
Fixes #3325

* Don't link

* Revert "Don't link"

This reverts commit 4fe847f91d9dd45cdc28a4984c4e44aad62a5de6.

* fix

* Revert "Revert "Don't link""

This reverts commit 856031b5a1fbad04ac218fa94ebf37dcd402f3ed.

* fix

* Deprecate this

* Better changelog entry
diff --git a/tests/test_x509_ext.py b/tests/test_x509_ext.py
index 8210b05..eaf5a51 100644
--- a/tests/test_x509_ext.py
+++ b/tests/test_x509_ext.py
@@ -1085,10 +1085,10 @@
             x509.load_pem_x509_certificate,
             backend
         )
-        with pytest.raises(x509.UnsupportedExtension) as exc:
-            cert.extensions
-
-        assert exc.value.oid == x509.ObjectIdentifier("1.2.3.4")
+        ext = cert.extensions.get_extension_for_oid(
+            x509.ObjectIdentifier("1.2.3.4")
+        )
+        assert ext.value.value == b"value"
 
     @pytest.mark.requires_backend_interface(interface=EllipticCurveBackend)
     def test_unsupported_extension(self, backend):