Fix some missing imports
diff --git a/Doc/lib/libcontextlib.tex b/Doc/lib/libcontextlib.tex
index 5f12af8..842c954 100644
--- a/Doc/lib/libcontextlib.tex
+++ b/Doc/lib/libcontextlib.tex
@@ -121,7 +121,9 @@
 
 And lets you write code like this:
 \begin{verbatim}
+from __future__ import with_statement
 from contextlib import closing
+import codecs
 
 with closing(codecs.open("foo", encoding="utf8")) as f:
     for line in f: