Add a definition (found in the Debian patches for 2.3.4)
diff --git a/Doc/tools/py2texi.el b/Doc/tools/py2texi.el
index b7155ac..8270c6e 100644
--- a/Doc/tools/py2texi.el
+++ b/Doc/tools/py2texi.el
@@ -211,6 +211,7 @@
     ("authoraddress" 1
      (progn (setq author-address (match-string 1 string)) ""))
     ("b" 1 "@w{\\1}")
+    ("backslash" 0 "@backslash{}")
     ("bf" 0 "@destroy")
     ("bifuncindex" 1 (progn (setq findex t) "@findex{\\1}"))
     ("C" 0 "C")