Fix failed tests, linter errors, and restructure (#477) * Switch development dependencies to Pipenv lockfile - add Pipfile and Dependabot-compatible Pipfile.lock - update lint, tox, CI, Travis, and Makefile workflows - support Python 3.10 through 3.14 - remove the pip-tools requirements lockfile - standardize linting, type checking, and tests * Streamline GitHub Actions CI workflow - consolidate duplicate lint and PR workflows - run strict linting, type checking, and tests - test against Python 3.10 through 3.14 - install dependencies from Pipfile.lock - add caching, timeouts, minimal permissions, and concurrency control - remove ignored failures and fragile changed-file logic