Stick with pytest < 4 to prevent internal error due to the use of deprecated pytest_namespace

[INFO]      [exec] collecting ... INTERNALERROR> Traceback (most recent call last):
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pytest-4.1.1-py2.7.egg/_pytest/main.py", line 203, in wrap_session
[INFO]      [exec] INTERNALERROR>     session.exitstatus = doit(config, session) or 0
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pytest-4.1.1-py2.7.egg/_pytest/main.py", line 242, in _main
[INFO]      [exec] INTERNALERROR>     config.hook.pytest_collection(session=session)
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pluggy-0.8.1-py2.7.egg/pluggy/hooks.py", line 284, in __call__
[INFO]      [exec] INTERNALERROR>     return self._hookexec(self, self.get_hookimpls(), kwargs)
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pluggy-0.8.1-py2.7.egg/pluggy/manager.py", line 68, in _hookexec
[INFO]      [exec] INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pluggy-0.8.1-py2.7.egg/pluggy/manager.py", line 62, in <lambda>
[INFO]      [exec] INTERNALERROR>     firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pluggy-0.8.1-py2.7.egg/pluggy/callers.py", line 208, in _multicall
[INFO]      [exec] INTERNALERROR>     return outcome.get_result()
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pluggy-0.8.1-py2.7.egg/pluggy/callers.py", line 81, in get_result
[INFO]      [exec] INTERNALERROR>     _reraise(*ex)  # noqa
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pluggy-0.8.1-py2.7.egg/pluggy/callers.py", line 187, in _multicall
[INFO]      [exec] INTERNALERROR>     res = hook_impl.function(*args)
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pytest-4.1.1-py2.7.egg/_pytest/main.py", line 252, in pytest_collection
[INFO]      [exec] INTERNALERROR>     return session.perform_collect()
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pytest-4.1.1-py2.7.egg/_pytest/main.py", line 466, in perform_collect
[INFO]      [exec] INTERNALERROR>     self.config.pluginmanager.check_pending()
[INFO]      [exec] INTERNALERROR>   File "nuxeo-python-client-master/.eggs/pluggy-0.8.1-py2.7.egg/pluggy/manager.py", line 251, in check_pending
[INFO]      [exec] INTERNALERROR>     % (name, hookimpl.plugin),
[INFO]      [exec] INTERNALERROR> PluginValidationError: unknown hook 'pytest_namespace' in plugin <module 'tests.conftest' from 'nuxeo-python-client-master/tests/conftest.py'>