Update syntax after dropping Python 3.6 support

[bhrutledge]

Following up on #869.

e.g., via:

fd -e py | xargs pyupgrade --py37-plus

Also, Twine can now take advantage of from __future__ import annotations, which means it can now use recent syntax like subscriptable generics and simpler union types. pyupgrade should handle most of that, as long as the __future__ import is present, which can be added with isort.

It might also be worth using typing_extensions to take advantage of newer typing PEPs.