bpo-42579: Make workaround for various versions of Sphinx more robust (GH-23662)

The solution in gh#python/cpython#13236 is too strict because it
effectively requires the use of Sphinx >= 2.0. It is not too difficult to
make the same solution more robust so it works with all normal versions
of Sphinx.

Doc/tools/extensions/pyspecific.py

@@ -394,6 +394,11 @@ class DeprecatedRemoved(Directive):
                                    translatable=False)
             node.append(para)
         env = self.state.document.settings.env
+        # deprecated pre-Sphinx-2 method
+        if hasattr(env, 'note_versionchange'):
+            env.note_versionchange('deprecated', version[0], node, self.lineno)
+        # new method
+        else:
             env.get_domain('changeset').note_changeset(node)
         return [node] + messages