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: I688a81fb0080bb0f1eb6538a93b2cd960129fc36
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -3,6 +3,8 @@ minversion = 4.28.0
 envlist = py3,pep8
 
 [testenv]
+setenv =
+  PYTHONDONTWRITEBYTECODE=1
 constraints =
   {env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 deps =
@@ -38,11 +40,12 @@ commands =
 commands = {posargs}
 
 [testenv:cover]
+setenv =
+  {[testenv]setenv}
+  PYTHON=coverage run --source debtcollector --parallel-mode
 deps =
   {[testenv]deps}
   coverage>=5.0 # Apache-2.0
-setenv =
-  PYTHON=coverage run --source debtcollector --parallel-mode
 commands =
   coverage erase
   {[testenv]commands}