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:

• [x] description (implemented by #174)
• [x] license (implemented by #180)
• [x] keywords
• [x] classifiers (implemented by #134?)

[kanghyojun]

I think it was finished on https://github.com/earlbread/nirum/commit/c348adeb4c443ed51f4902e2911140b6dafa873b. So I decided to close this issue.