Support PyQt5 compiled with SIP 5

When PyQt5 is compiled with SIP 5, the sip files are installed to ${python-site-packages}/PyQt5/bindings, so search for them there too.
This doesn't add support for compiling KF5 bindings themselves with sip5, that requires more work.

Differential Revision: https://phabricator.kde.org/D25972