Make _hook_into_pytest() work with pytest-4.1

This is however not backward compatible with pytest<4.1 since the
runner.CallInfo.from_call() was added in 4.1.0
pytest-dev/pytest@847eace

Fixes flexmock#27

flexmock.py

@@ -1224,8 +1224,7 @@ def call_runtest_hook(item, when, **kwargs):
             ret = saved(item, when, **kwargs)
             if when != 'call' and ret.excinfo is None:
                 return ret
-            teardown = runner.CallInfo(flexmock_teardown, when=when)
-            teardown.result = None
+            teardown = runner.CallInfo.from_call(flexmock_teardown, when=when)
             if ret.excinfo is not None and teardown.excinfo is None:
                 teardown.excinfo = ret.excinfo
             return teardown