Improve st2client homepage

[blag]

This PR improves the PyPI description of st2client by doing the following:

• Removes the redundant license specifier argument to setup() (see the official documentation on it here, and why this is a problem here)
• Adds the content of README.rst as the long description for the st2client package on PyPI
• Adds additional URLs to the project, which I think will be rendered in the sidebar of the PyPI project page, giving the links more visibility for users looking at the st2client package on PyPI

[nmaludy]

Should we set long_description_content_type to text/x-rst

https://packaging.python.org/guides/making-a-pypi-friendly-readme/

[blag]

According to the docs, that's not strictly necessary:

If a Description-Content-Type is not specified, then applications should attempt to render it as text/x-rst; charset=UTF-8 and fall back to text/plain if it is not valid rst.

[nmaludy]

Great work, thank you!

[blag]

Thanks for the review!