Remove backports.zoneinfo requirement

[`zoneinfo`][zoneinfo] is being added to Python as part of the 3.9 release
(current in beta). Once a stable release is available and this project has
upgraded, there will be no more need to use the [backport]. It can then be
removed from `requirements.in`.

[backport]: https://pypi.org/p/backports.zoneinfo
[zoneinfo]: https://docs.python.org/3.9/library/zoneinfo.html
