net-im/py-slixmpp: Add missing RUN_DEPENDS and fix some other issues

• Switch MASTER_SITES to CHEESESHOP as it ought to be the default for Python software.
• Adjust COMMENT to match the short description from setup.py.
• Add missing RUN_DEPENDS.
• Standardize test invocation.
• Update WWW to point to project's homepage instead of the Git repository.

Reported by: koobs
Reviewed by: koobs, krion
Approved by: koobs, krion (mentor)
Differential Revision: https://reviews.freebsd.org/D18684