Add `import datetime as dt` as a conventional alias

Summary

I propose adding "datetime": "dt" as an entry in the lint.flake8-import-conventions.aliases default.

CPython started recommending this alias from a change earlier this year by @hugovk: https://github.com/python/cpython/pull/145315

This was based on my 2019 blog post: https://adamj.eu/tech/2019/09/12/how-i-import-pythons-datetime-module/