Use warnings.warn and deprecation warning

sphinx/domains/python.py

@@ -496,10 +496,12 @@ def handle_signature(self, sig: str, signode: desc_signature) -> Tuple[str, str]
  sig_prefix = self.get_signature_prefix(sig)
  if sig_prefix:
  if type(sig_prefix) is str:
- logger.warning(
- "Python directive get_signature_prefix() returns a str ('{}')"
- " instead of a list of nodes (changed in 4.3).".format(sig_prefix),
- location=signode)
+ warnings.warn(
+ "Python directive method get_signature_prefix()"
+ " returning a string is deprecated."
+ " It must now return a list of nodes."
+ " Return value was '{}'.".format(sig_prefix),
+ RemovedInSphinx50Warning)
  signode += addnodes.desc_annotation(sig_prefix, '', # type: ignore
  nodes.Text(sig_prefix)) # type: ignore
  else: