CI update mypy pin

[MarcoGorelli]

There's a new mypy version out https://mypy-lang.blogspot.com/2021/02/mypy-0812-released.html - it doesn't look like the changes should affect type checking in pandas, but it's probably a good idea to stay up-to-date

Task is to update the mypy pin in environment.yml and requirements-dev.txt, run mypy pandas, and then open a PR if it all passes (if it doesn't, please report back)