Add simplify plugin to linting checks

[wren]

We recently added flakeheaven to our linting checks. This PR adds a flake8-simplify to look for more general issues with code complexity, and provide specific advice on how to fix those issues. Running this locally, we had a few issues already in the codebase, so this PR also fixes those issues so the new checks can start from a neutral state.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

[micahellison]

💯