add license to PKG_INFO metadata

[schumannd]

currently this package shows up in automated license checkers as UNKNOWN. I think this can be fixed by adding a license key to the [metadata] section in setup.cfg.

[jaraco]

The reason 'license' is not indicated is because

• the documentation states that the license field is intended for when Trove classifiers are insufficient.
• The Trove classifier is sufficient.
• Indicating it in the license field would be redundant and error-prone (possibly inconsistent).

Instead, the license checkers should infer the license from the LICENSE file, as other license checkers do.