Our previous Pylint version was 2.8, and I recently upgraded it to 2.16 in 041877b. To facilitate landing it, I disabled a bunch of checks. But most of them would be good to activate.

Working on this PR means:

1. Choose one of the checks you want to enable from here:

https://github.com/Qiskit/qiskit-terra/blob/4e3b283157b43687db4260cee6decf17fbb37608/.pylintrc#L77-L109

3. Delete it from .pylintrc
4. Run tox -e lint and fix all the issues until tox -e lint succeeds.

To make it easier for code reviewers, it's a good idea to activate 1 or 2 checks at a time.