4.0.0: Incompatible change to function signatures; useful commandline program.

Ruff's flake8-compatible diagnostic "flake8-boolean-trap" recommends making boolean parameters (such as the check_validity flag of several functions) kw-only. This requires an incompatible API change.

This also adds a leapsecond commandline program that can be used to inspect the leap second database and perform UTC/TAI conversions.

What's Changed

• Add a useful commandline program by @jepler in #14
• Enable ruff E, F, and D tests by @jepler in #15
• More ruff fixes by @jepler in #16

Full Changelog: 3.0.0...4.0.0