Add required generic type

pydoctor/epydoc/markup/restructuredtext.py

@@ -190,7 +190,7 @@ def report(self, error: nodes.system_message) -> None:
 
         self._errors.append(ParseError(msg, linenum, is_fatal))
 
-class _DocumentPseudoWriter(Writer):
+class _DocumentPseudoWriter(Writer[str]):
     """
     A pseudo-writer for the docutils framework, that can be used to
     access the document itself.  The output of C{_DocumentPseudoWriter}