Do not generate bytecode in tests

This is a known best practise during development, to avoid leaving
stale bytecode after a file is deleted.

Change-Id: I83a89536533f5c1eeb9a687e640802ace7059e1e
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -3,11 +3,12 @@ minversion = 4.28.0
 envlist = py3,pep8
 
 [testenv]
+setenv =
+  PYTHONDONTWRITEBYTECODE=1
 constraints =
   {env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 deps =
   -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
@@ -28,6 +29,7 @@ commands =
 
 [testenv:cover]
 setenv =
+  {[testenv]setenv}
   PYTHON=coverage run --source pycadf --parallel-mode
 deps =
   {[testenv]deps}