Update mypy to 1.16.0 (#14194)

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stubs/WTForms/@tests/stubtest_allowlist.txt

@@ -9,6 +9,14 @@ wtforms.fields.core.Field.__get__
 # to let mypy know that arbitrary attribute access is possible
 wtforms.meta.DefaultMeta.__getattr__
 
+# Should allow setting and deleting any attribute
+wtforms.Flags.__delattr__
+wtforms.Flags.__setattr__
+wtforms.fields.Flags.__delattr__
+wtforms.fields.Flags.__setattr__
+wtforms.fields.core.Flags.__delattr__
+wtforms.fields.core.Flags.__setattr__
+
 # Error: variable differs from runtime
 # ======================
 # _unbound_fields has some weird semantics: due to the metaclass it