python/trx: rename 'change_sign_of_dict_elements' to 'dict_toggle_sign'

The old name was quite long, resulting into cumbersome imports:

  from change_sign_of_dict_elements import change_sign_of_dict_elements

let's use a shorter variant:

  from dict_toggle_sign import dict_toggle_sign

Change-Id: Ie75e1d6e5e74c7c1cf34154633c1472e4b85dbb6
diff --git a/python/trx/__init__.py b/python/trx/__init__.py
index 372f46b..fda783e 100644
--- a/python/trx/__init__.py
+++ b/python/trx/__init__.py
@@ -26,4 +26,4 @@
 from fake_pm import fake_pm
 from radio_if_grc import radio_if_grc
 from radio_if import radio_if
-from change_sign_of_dict_elements import change_sign_of_dict_elements
+from dict_toggle_sign import dict_toggle_sign