use moto instead of tests/mock_boto3 for tests

[coyotemarin]

mrjob has historically mocked out various AWS services. Currently this code lives in tests/mock_boto3.

The moto library does basically the same thing. mrjob should probably try to move to moto, at least for fully mocked out interfaces like S3. This would eliminate the enormous hassle of adding new mocks, and allow people who are already using moto for testing to write mrjob tests.