devel/py-singledispatch: Add missing dependency (six)

• Add missing RUN_DEPENDS on six [1]
• Canonicalize, clarify COMMENT
• Limit USE_PYTHON version to <= 3.3 (Unnecessary in 3.4+)
• Enable architecture independence (NO_ARCH)
• Enable concurrent Python installation

PR: 213314 [1]
Submitted by: Kyle Evans <bsdports kyle-evans net> [1]
Approved by: python (maintainer)
MFH: 2016Q4