Consider updating pytest

[vreuter]

pytest is now on its next major version (4.x), so we should probably transition at some point.

[vreuter]

Tests fail on newest pytest, presumably due to significan API alteration with a new major version. Here's a sample from https://travis-ci.org/pepkit/peppy/jobs/479541192

==================================== ERRORS ====================================
_______ ERROR collecting tests/models/independent/test_ProjectContext.py _______
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/pluggy/hooks.py:284: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/pluggy/manager.py:68: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/pluggy/manager.py:62: in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/_pytest/python.py:225: in pytest_pycollect_makeitem
    res = list(collector._genfunctions(name, obj))
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/_pytest/python.py:405: in _genfunctions
    self.ihook.pytest_generate_tests(metafunc=metafunc)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/pluggy/hooks.py:284: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/pluggy/manager.py:68: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/pluggy/manager.py:62: in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/_pytest/python.py:132: in pytest_generate_tests
    metafunc.parametrize(*marker.args, **marker.kwargs)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/_pytest/python.py:903: in parametrize
    ids = self._resolve_arg_ids(argnames, ids, parameters, item=self.definition)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/_pytest/python.py:957: in _resolve_arg_ids
    ids = idmaker(argnames, parameters, idfn, ids, self.config, item=item)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/_pytest/python.py:1103: in idmaker
    for valindex, parameterset in enumerate(parametersets)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/_pytest/python.py:1093: in _idvalset
    for val, argname in zip(parameterset.values, argnames)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/_pytest/python.py:1063: in _idval
    six.raise_from(ValueError(msg), e)
../../../virtualenv/python2.7.14/lib/python2.7/site-packages/six.py:737: in raise_from
    raise value
E   ValueError: tests/models/independent/test_ProjectContext.py::ProjectContextTests::test_inclusion: error raised while trying to determine id of parameter 'expected_names' at position 0
E     IndexError: tuple index out of range

[khoroshevskyi]

It is an old issue, and peppy was updated to > Python3.8, and tests were rewritten. Closing..