py: Make tuple and list use mp_int_t/mp_uint_t.

Part of code cleanup, to resolve issue #50.
diff --git a/py/builtinimport.c b/py/builtinimport.c
index debf75f..fad9567 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -70,7 +70,7 @@
 
 STATIC mp_import_stat_t find_file(const char *file_str, uint file_len, vstr_t *dest) {
     // extract the list of paths
-    uint path_num = 0;
+    mp_uint_t path_num = 0;
     mp_obj_t *path_items;
 #if MICROPY_PY_SYS
     mp_obj_list_get(mp_sys_path, &path_num, &path_items);