At very long last land PyQt5 5.5.1 ports.
Add the required bits to Uses/pyqt.mk along with all the PyQt5 ports.
Thankfully this commit is mostly adding new ports, as the hard work was
already done in r403297 and r403662.
Huge kudos to Tobias Berner <email@example.com> and, most importantly,
Guido Falsi (madpilot@) for their initial work on these ports (see D2910 in
Phabricator for an earlier version of the PyQt5 patch set).