All classes now subclass object for better py3 compatibility

Fix #2147

testing/python/collect.py

@@ -880,10 +880,10 @@ class TestConftestCustomization(object):
             import sys, os, imp
             from _pytest.python import Module
 
-            class Loader:
+            class Loader(object):
                 def load_module(self, name):
                     return imp.load_source(name, name + ".narf")
-            class Finder:
+            class Finder(object):
                 def find_module(self, name, path=None):
                     if os.path.exists(name + ".narf"):
                         return Loader()

testing/python/fixture.py

@@ -2828,7 +2828,7 @@ class TestShowFixtures(object):
     def test_show_fixtures_indented_in_class(self, testdir):
         p = testdir.makepyfile(dedent('''
             import pytest
-            class TestClass:
+            class TestClass(object):
                 @pytest.fixture
                 def fixture1(self):
                     """line1

testing/python/metafunc.py

@@ -241,7 +241,7 @@ class TestMetafunc(object):
         """
         from _pytest.python import _idval
 
-        class TestClass:
+        class TestClass(object):
             pass
 
         def test_function():