attrs 17.3.0 breaks some of our tests

[mattsb42-aws]

Problem

The backwards incompatible change in attrs 17.3.0 broke some of our tests. This is apparently due to our use of a feature that was deprecated in 16.1.

Solution

The good news is that none of our actual client code uses this deprecated feature, so users should not be affected by this. Because of this, we just need to update our affected tests to move away from this access pattern.

[mattsb42-aws]

While I will be fixing these tests, we may also need to blacklist attrs==17.3.0: python-attrs/attrs#303

[mattsb42-aws]

To wrap up this issue, in #33 I fixed both problems identified above.