Don't look at user's AWS config when running tests

If you set up a different config file locally and then run tests, the
tests easily fail, and who knows what it's doing with user's credentials
:/

tox.ini

@@ -13,6 +13,9 @@ setenv =
 	DJANGO_SETTINGS_MODULE = tests.settings
 	PYTHONWARNINGS = always
 	PYTHONDONTWRITEBYTECODE = 1
+	# Use a non-existent file to prevent boto3 from loading
+	# any configuration from the user's environment
+	AWS_CONFIG_FILE = {toxinidir}/tests/no_such_file.conf
 commands = pytest --cov=storages tests/ {posargs}
 deps =
 	cryptography