Mark py2 function as no-cover on Python 3

Author: jaraco

importlib_metadata/_compat.py

@@ -70,7 +70,7 @@ def find_spec(*args, **kwargs):
     find_module = find_spec
 
 
-def py2_message_from_string(text):
+def py2_message_from_string(text):  # nocoverpy3
     # Work around https://bugs.python.org/issue25545 where
     # email.message_from_string cannot handle Unicode on Python 2.
     io_buffer = io.StringIO(text)