stevedore.tests.test_cache.TestCache.test_disable_caching_file test fails

[Michał Górny]

When running the test suite via tox (e.g. tox -e py38), I get the following test failure:

```
stevedore.tests.test_cache.TestCache.test_disable_caching_file
--------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/tmp/stevedore/stevedore/tests/test_cache.py", line 47, in test_disable_caching_file
    self.assertFalse(sot._disable_caching)

      File "/usr/lib/python3.8/unittest/case.py", line 759, in assertFalse
    raise self.failureException(msg)

    AssertionError: True is not false
```