fix or supress the ResourceWarning in the tests

[eviljeff]

Under python3 we get many (many!) warnings in the logs such as

.tox/devhub/lib/python3.6/site-packages/mock/mock.py:1986
/home/travis/build/mozilla/addons-server/.tox/devhub/lib/python3.6/site-packages/mock/mock.py:1986: ResourceWarning: unclosed <socket.socket fd=18, family=AddressFamily.AF_INET, type=2049, proto=0, laddr=('127.0.0.1', 39844), raddr=('127.0.0.1', 11211)>
setattr(_type, entry, MagicProxy(entry, self))

https://github.com/kennethreitz/requests/issues/3912 and https://github.com/kennethreitz/requests/issues/1882 discuss it. I skim read them and I'm not sure it's fixable (or worth the effort to fix something that only occurs in our tests) mobolic/facebook-sdk#364 has an example of how to suppress the warnings

[stale]

This issue has been automatically marked as stale because it has not had recent activity. If you think this bug should stay open, please comment on the issue with further details. Thank you for your contributions.