python: add file encoding spec to run_setup.py

Newer 'mutagen' versions have a test suite that trips over missing
encoding in this file. The file is copied to source trees at build time
with the name nix_run_setup.py.

pkgs/development/python-modules/generic/run_setup.py

@@ -1,3 +1,5 @@
+# -*- coding: utf-8 -*-
+
 import setuptools
 import tokenize