test: fix None comparisons (#1963)

`flake8` will eventually complain about this when the `flakeheaven` baseline
is invalidated.

test/jsonld/test_context.py

@@ -196,7 +196,7 @@ def test_ignore_base_remote_context():
     ctx_url = "http://example.org/remote-base.jsonld"
     SOURCES[ctx_url] = {"@context": {"@base": "/remote"}}
     ctx = Context(ctx_url)
-    assert ctx.base == None
+    assert ctx.base is None
 
 
 @_expect_exception(errors.RECURSIVE_CONTEXT_INCLUSION)

test/test_namespace/test_namespace.py

@@ -267,19 +267,19 @@ def test_expand_curie_exception_messages(self) -> None:
         g = Graph()
 
         with pytest.raises(TypeError) as e:
-            assert g.namespace_manager.expand_curie(URIRef("urn:example")) == None
+            assert g.namespace_manager.expand_curie(URIRef("urn:example")) is None
         assert str(e.value) == "Argument must be a string, not URIRef."
 
         with pytest.raises(TypeError) as e:
-            assert g.namespace_manager.expand_curie(Literal("rdf:type")) == None
+            assert g.namespace_manager.expand_curie(Literal("rdf:type")) is None
         assert str(e.value) == "Argument must be a string, not Literal."
 
         with pytest.raises(TypeError) as e:
-            assert g.namespace_manager.expand_curie(BNode()) == None
+            assert g.namespace_manager.expand_curie(BNode()) is None
         assert str(e.value) == "Argument must be a string, not BNode."
 
         with pytest.raises(TypeError) as e:
-            assert g.namespace_manager.expand_curie(Graph()) == None
+            assert g.namespace_manager.expand_curie(Graph()) is None
         assert str(e.value) == "Argument must be a string, not Graph."
 
     @pytest.mark.parametrize(