Clean up imports

[sajith]

Issue is #184. These are the changes:

• Remove all unused imports.
• Avoid some wildcard imports, and explicitly import classes instead.
• Import some modules that are used rarely (because they are used by __main__ of those modules, which we rarely invoke)
• Remove an unused variable.

These were found by the ruff linter, and I am considering adding ruff checks as a CI step. Ruff reported some more errors, about which I will follow-up separately.

This PR also carries an unrelated change caused by coveralls triggering a goofy error: omit test modules from coverage measurement. It causes the coverage report to drop, but we will also get a more "honest" coverage report. I am planning on working on improving test coverage, so this would help.

[coveralls]

coverage: 36.146% (-9.2%) from 45.352% when pulling 216f1c3d94b0e158dd5d60b6e4de7ea4eb2bae73 on 184.clean-up-imports into 81ac2adf0e98ba13702a53e9fe0a2b8ebe9c4e6e on main.