Using Python linter flake8 on the utils/lit reveals several linter
warnings designated "F401: Unused import". Fix or silence these
Some of these unused imports are legitimate, while some are part of lit's API.
For example, users of lit expect to be able to access lit.formats.ShTest in
their lit.cfg, despite the module hierarchy for that symbol actually being
lit.formats.shtest.ShTest. To silence linter errors for these lines,
include a "noqa" directive.