Include AUTHORS.txt in pip's wheels

Wheels already include `LICENSE.txt`, which refers to `AUTHORS.txt`, so
`AUTHORS.txt` should be included too.

Conveniently, both `LICENSE.txt` and `AUTHORS.txt` are included by setuptools
by default, so all we need is to remove the manual `license_file` setting.

This also fixes a build-time warning, since `license_file` is deprecated
in favor of `license_files`.

setup.cfg

@@ -111,6 +111,3 @@ exclude_lines =
     pragma: no cover
     # This excludes typing-specific code, which will be validated by mypy anyway.
     if TYPE_CHECKING
-
-[metadata]
-license_file = LICENSE.txt