Re-enable checks in .pylintrc and resolve the pylint warnings

[mhucka]

Pylint 3 has many more checks than 2.x did, and in converting from Pylint 2.x to 3.x, I had to turn off many checks because there were so many warnings and time was limited. We need to do another pass through that, systematically re-enabling checks in .pylintrc and correcting the pylint warnings that result.