Ethereum serpent version conflict error

Question

I have installed pythereum and ethereum serpent. Now I try to run the following code:

pytest -m test_contracts.py

But i get this error:

INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/main.py", line 96, in wrap_session
INTERNALERROR>     session.exitstatus = doit(config, session) or 0
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/main.py", line 130, in _main
INTERNALERROR>     config.hook.pytest_collection(session=session)
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/vendored_packages/pluggy.py", line 724, in __call__
INTERNALERROR>     return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/vendored_packages/pluggy.py", line 338, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/vendored_packages/pluggy.py", line 333, in <lambda>
INTERNALERROR>     _MultiCall(methods, kwargs, hook.spec_opts).execute()
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/vendored_packages/pluggy.py", line 596, in execute
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/main.py", line 139, in pytest_collection
INTERNALERROR>     return session.perform_collect()
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/main.py", line 592, in perform_collect
INTERNALERROR>     config=self.config, items=items)
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/vendored_packages/pluggy.py", line 724, in __call__
INTERNALERROR>     return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/vendored_packages/pluggy.py", line 338, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/vendored_packages/pluggy.py", line 333, in <lambda>
INTERNALERROR>     _MultiCall(methods, kwargs, hook.spec_opts).execute()
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/vendored_packages/pluggy.py", line 596, in execute
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/mark.py", line 84, in pytest_collection_modifyitems
INTERNALERROR>     if not matchmark(colitem, matchexpr):
INTERNALERROR>   File "/usr/local/lib/python2.7/site-packages/_pytest/mark.py", line 124, in matchmark
INTERNALERROR>     return eval(markexpr, {}, MarkMapping(colitem.keywords))
INTERNALERROR>   File "<string>", line 1, in <module>
INTERNALERROR> AttributeError: 'bool' object has no attribute 'py'