Python 3.9 removed ``asyncio.Task.all_tasks`` so I switched to using
``asyncio.all_tasks`` instead. The ``all_tasks`` function was added
in 3.7 and ``Task.all_tasks`` was deprecated then.
- Drop support for all versions of tornado prior to 5.0. This negates
the need for version-aware conditionals that previous existed.
- Update tox to test Tornado 5.0 and the unpinned latest version.