You’re welcome! Sorry it was so much trouble to get working, but I’m glad we figured it out! FYI Pants 1.24 adds support for Flake8. In a later Pants version,

python.checks

gets deprecated in favor of Flake8.

python.checks

was a weird custom version of Flake8 with fewer checks and without support for Flake8 plugins. See https://pants.readme.io/docs/python-linters-and-formatters for how to activate Flake8