Python 2.7: increase priority - fixup

From the manual:

> This attribute should be a number, with a higher value denoting a
lower priority. The default priority is 0.

Just passing -5 or -10 wasn't sufficient, so let's make it -100.

pkgs/development/interpreters/python/cpython/2.7/default.nix

@@ -223,6 +223,6 @@ in stdenv.mkDerivation {
       maintainers = with stdenv.lib.maintainers; [ chaoflow domenkozar ];
       # Higher priority than Python 3.x so that `/bin/python` points to `/bin/python2`
       # in case both 2 and 3 are installed.
-      priority = 5;
+      priority = -100;
     };
   }