Sort out interactions with local conftest.py

[ev-br]

Trying to run $ pytest -pno:doctest --doctest-modules ~/repos/numpy/numpy/linalg/linalg.py -v -s results in

ImportError while loading conftest '/home/br/repos/numpy/numpy/conftest.py'.
/home/br/repos/numpy/numpy/__init__.py:129: in <module>
    raise ImportError(msg) from e
E   ImportError: Error importing numpy: you should not try to import numpy from
E           its source directory; please exit the numpy source tree, and relaunch
E           your python interpreter from there.