add missing description of os.extsep

Doc/lib/libos.tex

@@ -1643,6 +1643,11 @@ set to \character{/} on Windows systems where \code{sep} is a
 backslash.
 \end{datadesc}
 
+\begin{datadesc}{extsep}
+The character which separates the base filename from the extension,
+e.g. \character{.} in \code{os.py}.
+\end{datadesc}
+
 \begin{datadesc}{pathsep}
 The character conventionally used by the operating system to separate
 search patch components (as in \envvar{PATH}), such as \character{:} for