devel/py-mypy: update to 0.521

ChangeLog:

• Fine-graned control of any type
• setattr support
• Hash-based incremental cache
• NamedTuple methods
• Improved error messages

See http://mypy-lang.blogspot.co.za/2017/07/mypy-0520-released.html and
http://mypy-lang.blogspot.co.za/2017/07/mypy-0521-released.html

PR: 221230
Approved by: Florian Limberger (flo@snakeoilproductions.net)