Adopt Python 3.7+ syntax

[bhrutledge]

Via pyupgrade --py37-plus. Closes #881.

I started another branch to adopt from __future__ import annotations, but found that support for abbreviated Union types won't be fully supported at runtime (e.g. in type aliases and cast) until Python 3.10. Rather than having multiple idioms for the same concept (e.g. str | None and Optional[str]), I found a workaround for the one instance where __future__ annotations was used.

[sigmavirus24]

Why did this become quoted?

[bhrutledge]

That's what I was referring to here:

I found a workaround for the one instance where __future__ annotations was used.

The quotes avoid a syntax error in Python < 3.9 (which introduces typed generics), similar to forward references:

twine/twine/settings.py

Lines 276 to 281 in a0ba32d

	def from_argparse(cls, args: argparse.Namespace) -> "Settings":
	"""Generate the Settings from parsed arguments."""
	settings = vars(args)
	settings["repository_name"] = settings.pop("repository")
	settings["cacert"] = settings.pop("cert")
	return cls(**settings)

My understanding is that __future__ annotations treats type annotations as strings, so this mirrors that behavior. It feels more consistent with the rest of the project, and avoids pyupgrade making other typing rewrites when that import is present.