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

Fix invalid Python file encoding "utf8"

testing/python/approx.py

@@ -1,4 +1,4 @@
-# encoding: utf-8
+# -*- coding: utf-8 -*-
 import doctest
 import operator
 import sys

testing/python/collect.py

@@ -1244,7 +1244,7 @@ def test_syntax_error_with_non_ascii_chars(testdir):
     """
     testdir.makepyfile(
         u"""
-    # -*- coding: UTF-8 -*-
+    # -*- coding: utf-8 -*-
 
     ☃
     """

testing/python/integration.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 import pytest
 from _pytest import python
 from _pytest import runner

testing/python/raises.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 import sys
 
 import six

testing/python/setup_only.py

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

testing/python/setup_plan.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 def test_show_fixtures_and_test(testdir):
     """ Verifies that fixtures are not executed. """
     p = testdir.makepyfile(