Use warnings.warn and deprecation warning

Author: jakobandersen

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: