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: I090d3d0e1cbc1cb72eb2b75a6629e68015cbb90b
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -3,6 +3,8 @@ minversion = 4.28.0
 envlist = py3,pep8,docs
 
 [testenv]
+setenv =
+  PYTHONDONTWRITEBYTECODE=1
 constraints =
   {env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 deps =
@@ -37,6 +39,7 @@ commands =
 
 [testenv:cover]
 setenv =
+  {[testenv]setenv}
   PYTHON=coverage run --source stevedore --parallel-mode
 deps =
   {[testenv]deps}