alias_for_import_name(): Dueling compiler warnings ;-)

Squash new warnings from VC 7.1 about mixing signed and
unsigned types in comparisons.  I can see why `len` was
changed to size_t here, but don't see why `i` was also
changed.  Change `i` back to int.

Python/ast.c

@@ -2030,7 +2030,8 @@ alias_for_import_name(struct compiling *c, const node *n)
                 return alias(NEW_IDENTIFIER(CHILD(n, 0)), NULL, c->c_arena);
             else {
                 /* Create a string of the form "a.b.c" */
-                size_t i, len;
+				int i;
+                size_t len;
                 char *s;
 
                 len = 0;