tests: replace "from util import *" by explicit imports

tests/test_theming.py

@@ -12,10 +12,10 @@
 import os
 import zipfile
 
-from util import *
-
 from sphinx.theming import Theme, ThemeError
 
+from util import with_app, raises
+
 
 @with_app(confoverrides={'html_theme': 'ziptheme',
                          'html_theme_options.testopt': 'foo'})