Add a link to the mxNumber package.

Doc/lib/libmpz.tex

@@ -103,4 +103,9 @@ An mpz-number has one method:
             This project is building new numeric types to allow
             arbitrary-precision arithmetic in Python.  Their first
             efforts are also based on the GNU MP library.}
+
+  \seetitle[http://www.egenix.com/files/python/mxNumber.html]{mxNumber
+            --- Extended Numeric Types for Python}{Another wrapper
+            around the GNU MP library, including a port of that
+            library to Windows.}
 \end{seealso}