Use tox for tests
Tox reduces the risk of testing against locally installed version(s) by using a virtualenv for every test, and also allows easily testing on multiple platforms (Windows, Linux, but also Python 3.5 vs. 3.6). One can also easily add formatting tests, coverage tests, etc. using tox.
See e.g. https://gitlab.com/deltares/skeletonizer for example on how to use tox.