python39: hardcode path to tzdata

Python 3.9 has a new module `zoneinfo` which requires tzdata. By default
it searches TZPATH for folders containing `zoneinfo`.

This commit makes the dependency on tzdata pure.

pkgs/development/interpreters/python/default.nix

@@ -199,6 +199,7 @@ in {
     gdbm = null;
     sqlite = null;
     configd = null;
+    tzdata = null;
     stripConfig = true;
     stripIdlelib = true;
     stripTests = true;