tox: remove explicit custom python path

Removes the custom Python path explicitly set in the tox configuration
file. This should resolve situations [1] where pip may not install
required packages (and dependencies) into target virtualenv since they
are already provided in PYTHONPATH (see also [2]).

[1]: #45
[2]: tox-dev/tox#330

Signed-off-by: James Knight <[email protected]>

Author: jdknight

tox.ini

@@ -4,8 +4,6 @@ skip_missing_interpreters = true
 
 [testenv]
 deps = -r{toxinidir}/requirements_dev.txt
-setenv =
-    PYTHONPATH = {toxinidir}:{toxinidir}/pluralsight
 changedir=test
 commands =
     python -m test_builder