More Python metadata

[dahlia]

Nirum currently generates Python packages with minimum required metadata such as author, version, and name. There are more than these fields:

• description (implemented by Add description field to package metadata #174)
• license (implemented by Add license field to package metadata #180)
• keywords
• classifiers (implemented by [WIP] #134?)

[kanghyojun]

I think it was finished on earlbread@c348ade. So I decided to close this issue.