spy: handle AttributeError with __getattribute__

I have noticed that `spy` did not work on functions that are not defined
in the class itself, but a parent class.  This patch fixes this.

pytest_mock.py

@@ -60,11 +60,15 @@ def spy(self, obj, name):
         # Can't use autospec classmethod or staticmethod objects
         # see: https://bugs.python.org/issue23078
         if inspect.isclass(obj):
-            # bypass class descriptor:
+            # Bypass class descriptor:
             # http://stackoverflow.com/questions/14187973/python3-check-if-method-is-static
-            value = obj.__getattribute__(obj, name)
-            if isinstance(value, (classmethod, staticmethod)):
-                autospec = False
+            try:
+                value = obj.__getattribute__(obj, name)
+            except AttributeError:
+                pass
+            else:
+                if isinstance(value, (classmethod, staticmethod)):
+                    autospec = False
 
         result = self.patch.object(obj, name, side_effect=method,
                                    autospec=autospec)

test_pytest_mock.py

@@ -211,6 +211,27 @@ def bar(self, arg):
     assert spy.call_args_list == calls
 
 
+@skip_pypy
+def test_instance_method_by_subclass_spy(mocker):
+    from pytest_mock import mock_module
+
+    class Base(object):
+
+        def bar(self, arg):
+            return arg * 2
+
+    class Foo(Base):
+        pass
+
+    spy = mocker.spy(Foo, 'bar')
+    foo = Foo()
+    other = Foo()
+    assert foo.bar(arg=10) == 20
+    assert other.bar(arg=10) == 20
+    calls = [mock_module.call(foo, arg=10), mock_module.call(other, arg=10)]
+    assert spy.call_args_list == calls
+
+
 @skip_pypy
 def test_class_method_spy(mocker):
     class Foo(object):
@@ -225,6 +246,24 @@ def bar(cls, arg):
     spy.assert_called_once_with(arg=10)
 
 
+@skip_pypy
+def test_class_method_subclass_spy(mocker):
+    class Base(object):
+
+        def bar(self, arg):
+            return arg * 2
+
+    class Foo(Base):
+        pass
+
+
+    spy = mocker.spy(Foo, 'bar')
+    foo = Foo()
+    assert foo.bar(arg=10) == 20
+    Foo.bar.assert_called_once_with(foo, arg=10)
+    spy.assert_called_once_with(foo, arg=10)
+
+
 @skip_pypy
 def test_class_method_with_metaclass_spy(mocker):
     class MetaFoo(type):
@@ -258,6 +297,24 @@ def bar(arg):
     spy.assert_called_once_with(arg=10)
 
 
+@skip_pypy
+def test_static_method_subclass_spy(mocker):
+    class Base(object):
+
+        @staticmethod
+        def bar(arg):
+            return arg * 2
+
+
+    class Foo(Base):
+        pass
+
+    spy = mocker.spy(Foo, 'bar')
+    assert Foo.bar(arg=10) == 20
+    Foo.bar.assert_called_once_with(arg=10)
+    spy.assert_called_once_with(arg=10)
+
+
 @contextmanager
 def assert_traceback():
     """