py-pyqt4: fix for the depends_lib reset caused by the Python PortGroup

[Marko Käning]

Turns out there is a double dep on qt5-qtbase now:
---
$ port deps py\*-pyqt5
Full Name: py-pyqt5 @5.6_3
Build Dependencies:   pkgconfig
Library Dependencies: py27-pyqt5, qt5-qtbase
--
Full Name: py27-pyqt5 @5.6_3
Build Dependencies:   pkgconfig
Library Dependencies: qt5-qtbase, python27, qt5-qtbase, py27-sip, dbus-python27
--
Full Name: py34-pyqt5 @5.6_3
Build Dependencies:   pkgconfig
Library Dependencies: qt5-qtbase, python34, qt5-qtbase, py34-sip, dbus-python34
--
Full Name: py35-pyqt5 @5.6_3
Build Dependencies:   pkgconfig
Library Dependencies: qt5-qtbase, python35, qt5-qtbase, py35-sip, dbus-python35
---

I closed https://trac.macports.org/ticket/53090 obviously too quickly. :(