Update pyqt to 5.6.2

[adridg]

Summary

Update PyQT from 5.5.1 to 5.6.2 to match the Qt updates that have
already landed in the ports tree.

Test Plan

Builds on 10-STABLE amd64, also parallel with PyQt4. Needs runtime
testing through PyQT applications (Scintilla, and there's an
audio tagger thingy).

[tcberner]

Looks good to me.

[rakuco]

Looks fine to me too; in my memory PyQt updates used to be bigger and more difficult to handle. Did you file an exp-run request for this? While the diff looks small, we had several issues with PyQt updates in the past.