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.

IPython/external/qt_loaders.py

@@ -57,11 +57,11 b' def commit_api(api):'
     if api == QT_API_PYSIDE:
         ID.forbid('PyQt4')
         ID.forbid('PyQt5')
-    elif api == QT_API_PYQT:
+    elif api == QT_API_PYQT5:
         ID.forbid('PySide')
-        ID.forbid('PyQt5')
-    else:
         ID.forbid('PyQt4')
+    else:   # There are three other possibilities, all representing PyQt4
+        ID.forbid('PyQt5')
         ID.forbid('PySide')
 
 
@@ -241,7 +241,7 b' def load_qt(api_options):'
     ----------
     api_options: List of strings
         The order of APIs to try. Valid items are 'pyside',
-        'pyqt', 'pyqt5' and 'pyqtv1'
+        'pyqt', 'pyqt5', 'pyqtv1' and 'pyqtdefault'
 
     Returns
     -------