[1.9.x] Declared Sphinx extensions safe for parallel reading.

Backport of 03306a187e from master

docs/_ext/djangodocs.py

@@ -63,6 +63,7 @@ def setup(app):
                  man=(visit_snippet_literal, depart_snippet_literal),
                  text=(visit_snippet_literal, depart_snippet_literal),
                  texinfo=(visit_snippet_literal, depart_snippet_literal))
+    return {'parallel_read_safe': True}
 
 
 class snippet_with_filename(nodes.literal_block):

docs/_ext/ticket_role.py

@@ -36,3 +36,4 @@ def ticket_role(name, rawtext, text, lineno, inliner, options=None, content=None
 def setup(app):
     app.add_config_value('ticket_url', None, 'env')
     app.add_role('ticket', ticket_role)
+    return {'parallel_read_safe': True}