[Bug #1585690] Note that line_num was added in Python 2.5

Doc/lib/libcsv.tex

@@ -345,6 +345,7 @@ A read-only description of the dialect in use by the parser.
 \begin{memberdesc}[csv reader]{line_num}
  The number of lines read from the source iterator. This is not the same
  as the number of records returned, as records can span multiple lines.
+ \versionadded{2.5}
 \end{memberdesc}