Use flake8 instead pylint

Description

pylint does not play well with newer versions of gprcio and pydantic, as well as causing some unfortunate pins in the dependency lists. The change replaces pylint with flake8 as the main linting tool, which covers some of the missing cases found in black, isort, and mypy. The previous useful checks that pylint was performing have already been moved to this tooling.

Additonally, I've added flake8 usage to the tests, which has uncovered a number of small errors that are addressed by this change.