Fix invalid Python file encoding "utf8" (#5252)

Fix invalid Python file encoding "utf8"

src/_pytest/config/__init__.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 """ command line options, ini-file and conftest.py processing. """
 from __future__ import absolute_import
 from __future__ import division

src/_pytest/config/argparsing.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 import argparse
 import warnings
 

src/_pytest/config/exceptions.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 class UsageError(Exception):
     """ error in pytest usage or invocation"""
 

src/_pytest/config/findpaths.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 import os
 
 import py