[cross-project-tests] Use "is" instead of "==" to check for None (#94016)

From PEP8
(https://peps.python.org/pep-0008/#programming-recommendations):

> Comparisons to singletons like None should always be done with is or
is not, never the equality operators.
diff --git a/cross-project-tests/lit.cfg.py b/cross-project-tests/lit.cfg.py
index 9935fe6..c2a8bce 100644
--- a/cross-project-tests/lit.cfg.py
+++ b/cross-project-tests/lit.cfg.py
@@ -51,7 +51,7 @@
 
 def get_required_attr(config, attr_name):
     attr_value = getattr(config, attr_name, None)
-    if attr_value == None:
+    if attr_value is None:
         lit_config.fatal(
             "No attribute %r in test configuration! You may need to run "
             "tests from your build directory or add this attribute "