Fix Qt loader commit_api() for 'pyqtv1' or 'pyqtdefault'

Our Qt loaders have three options representing PyQt - pyqt (with v2
strings API), pyqtv1 (v1 ditto), and pyqtdefault, which is v2 on Python
3 only. When loading pyqtv1 or pyqtdefault, the code to deny imports of
other bindings would assume that api != pyqt meant we were using pyqt5,
and forbid future imports of PyQt4.

This changes the logic around so PyQt4 is the else: case, catching all
three possible names for it. PyQt5 and PySide have, so far, only one
name each.
Merge pull request #6730 from takluyver/qt-loaders-commit-fix

Fix Qt loader commit_api() for 'pyqtv1' or 'pyqtdefault'