Test two newest pytest minor versions

[hugovk]

To ensure compatibility with newer versions.

Pytest 6.1.0 currently fails due to #128.

Does this need a CHANGES.rst entry?

[icemac]

There should be a change log entry and a maximum version in setup.py until #128 is fixed.

[icemac]

Maybe also allowing 6.1 jobs to fail in Travis.

[hugovk]

From #127 (comment):

The current policy is to test against the latest 5 feature versions of pytest. So we decided to drop the older versions to reduce this list at least a bit. That means we still run 25 jobs on TravisCI.

With this change, should pytest 5.0 and 5.1 drop off the list?

[sallner]

@hugovk Thank you for submitting this PR. As there are so many PRs open partially depending on each other I have added another one #138 which builds on top of the current master. pytest 6.1.0 should be supported this way and we can close this PR.