Add "suspicious" builder which finds leftover markup in the HTML files.

Patch by Gabriel Genellina.
diff --git a/Doc/tools/sphinxext/pyspecific.py b/Doc/tools/sphinxext/pyspecific.py
index d389243..c3268e9 100644
--- a/Doc/tools/sphinxext/pyspecific.py
+++ b/Doc/tools/sphinxext/pyspecific.py
@@ -92,6 +92,9 @@
         finally:
             f.close()
 
+# Support for checking for suspicious markup
+
+import suspicious
 
 # Support for documenting Opcodes
 
@@ -116,5 +119,6 @@
 def setup(app):
     app.add_role('issue', issue_role)
     app.add_builder(PydocTopicsBuilder)
+    app.add_builder(suspicious.CheckSuspiciousMarkupBuilder)
     app.add_description_unit('opcode', 'opcode', '%s (opcode)',
                              parse_opcode_signature)