Revert "use lambda in closure" to fix doctest

This reverts commit 418cf40.

Author: jdemeyer

cypari2/closure.pyx

@@ -139,7 +139,8 @@ cpdef Gen objtoclosure(f):
     Examples:
 
     >>> from cypari2.closure import objtoclosure
-    >>> mul = objtoclosure(lambda i,j: i*j)
+    >>> def pymul(i,j): return i*j
+    >>> mul = objtoclosure(pymul)
     >>> mul
     (v1,v2,v3,v4,v5)->call_python(v1,v2,v3,v4,v5,...)
     >>> mul.type()