fix pyspecific extensions that were broken by Sphinx's grand renaming

Doc/tools/sphinxext/pyspecific.py

@@ -48,7 +48,7 @@ from pprint import pformat
 from docutils.io import StringOutput
 from docutils.utils import new_document
 from sphinx.builder import Builder
-from sphinx.textwriter import TextWriter
+from sphinx.writers.text import TextWriter
 
 class PydocTopicsBuilder(Builder):
     name = 'pydoc-topics'