bucket.

- Also, replaced dateutil with pytz.
- And use prefix instead of regex (as does S3).
- Added more sanity checks (like is the file timestamp from the future,
empty files, etc.).

- Use Pipenv for local development tasks in a virtualenv.
- Replace fabfile with Pipfile "scripts".
- Update .gitignore.
- Add lots of checks using pre-commit.
- Format code using Black.
- Use PEP 396 version tracking.
- Use bumpversion for tagging new versions.
- Fix issues found with Flake8, pylint.