Python code modernization, 3/n (#1157)

* Remove __metaclass__ = type.

for i in $(grep -REl '__metaclass__ = type' plugins/ tests/); do
  sed -e '/^__metaclass__ = type/d' -i $i;
done

* Remove super arguments, and stop inheriting from object.

plugins/doc_fragments/_attributes.py

@@ -8,7 +8,7 @@
 from __future__ import annotations
 
 
-class ModuleDocFragment(object):
+class ModuleDocFragment:
 
     # Standard documentation fragment
     DOCUMENTATION = r"""