Merge in qtconsole/qt.py API-loading logic.

IPython.external.qt was moved to qtconsole.qt, but was still imported
resulting in ImportErrors when trying to use Qt. This commit brings
the logic back into this file. Deferring to qt.py was a bit silly because
both qt.py and qt_for_kernel.py do basically the same thing but in
slightly different ways. Moreover, the assumption of qt.py was that if
the QT_API environment variable is set, we must be in an ETS environment
and need to comply by the ETS import logic. This is not valid, as
ipython sets the QT_API environment variable to communicate intent as to
Qt version, in IPython/lib/inputhook.py:385
@inputhook_manager.register('qt5')
class Qt5InputHook(Qt4InputHook):
    def enable(self, app=None):
        os.environ['QT_API'] = 'pyqt5'
        return Qt4InputHook.enable(self, app)

Now the logic is centralized.

IPython/external/qt_for_kernel.py

@@ -35,9 +35,12 b' import os'
 import sys
 
 from IPython.utils.version import check_version
-from IPython.external.qt_loaders import (load_qt, QT_API_PYSIDE,
-                                         QT_API_PYQT, QT_API_PYQT_DEFAULT,
-                                         loaded_api)
+from IPython.external.qt_loaders import (load_qt, loaded_api, QT_API_PYSIDE,
+                                         QT_API_PYQT, QT_API_PYQT5,
+                                         QT_API_PYQTv1, QT_API_PYQT_DEFAULT)
+
+_qt_apis = (QT_API_PYSIDE, QT_API_PYQT, QT_API_PYQT5, QT_API_PYQTv1,
+            QT_API_PYQT_DEFAULT)
 
 #Constraints placed on an imported matplotlib
 def matplotlib_options(mpl):
@@ -68,16 +71,15 b' def get_options():'
         #1.0.1 only supports PyQt4 v1
         return [QT_API_PYQT_DEFAULT]
 
-    if os.environ.get('QT_API', None) is None:
+    qt_api = os.environ.get('QT_API', None)
+    if qt_api is None:
         #no ETS variable. Ask mpl, then use either
         return matplotlib_options(mpl) or [QT_API_PYQT_DEFAULT, QT_API_PYSIDE]
-
-    #ETS variable present. Will fallback to external.qt
-    return None
+    elif qt_api not in _qt_apis:
+        raise RuntimeError("Invalid Qt API %r, valid values are: %r" %
+                           (qt_api, ' ,'.join(_qt_apis)))
+    else:
+        return [qt_api]
 
 api_opts = get_options()
-if api_opts is not None:
-    QtCore, QtGui, QtSvg, QT_API = load_qt(api_opts)
-
-else: # use ETS variable
-    from IPython.external.qt import QtCore, QtGui, QtSvg, QT_API
+QtCore, QtGui, QtSvg, QT_API = load_qt(api_opts)