Fix nightlies take two (#525)

The previous fix in #522 to ignore the spurious error being raise by our
nightlies fixes tf-nightly, but broke environments with tf < 2.10 as the
error symbol did not exists.

Let's match the error message, rather than the class, to work around
this.

setup.cfg

@@ -7,7 +7,8 @@ filterwarnings =
     ignore::PendingDeprecationWarning
     ignore::FutureWarning
     ignore::UserWarning
-    ignore::keras.saving.legacy.serialization.CustomMaskWarning
+    # Ignore a spurious warning on tf-nightly related to save model changes.
+    ignore:Custom mask layers require a config
 
 addopts=-vv