[ci] Fix ci's tox import errors

Since the last update of tox (3.10.0), it seems that we cannot pass the `tox tests` because of import errors...so we set the `PYTHONPATH` environment variable for our tox tests so the test can find the modules

Author: opacam

tox.ini

@@ -10,6 +10,8 @@ deps =
 # makes it possible to override pytest args, e.g.
 # tox -- tests/test_graph.py
 commands = pytest {posargs:tests/}
+setenv =
+    PYTHONPATH={toxinidir}
 
 [testenv:pep8]
 deps = flake8