Backport PR #8829: Qt5 fix

Fix for issue  #8757.

The problem was that IPython.external.qt was moved to qtconsole.qt, but IPython.external.qt_for_kernel was still trying to import IPython.external.qt.

Rather than copying the qt.py file back to external, this PR merges the logic back into qt_for_kernel.py. 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, it 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`
...

IPython/external/qt_for_kernel.py

@@ -1,83 +1,91 b''
 """ Import Qt in a manner suitable for an IPython kernel.
 
 This is the import used for the `gui=qt` or `matplotlib=qt` initialization.
 
 Import Priority:
 
-if Qt4 has been imported anywhere else:
+if Qt has been imported anywhere else:
    use that
 
 if matplotlib has been imported and doesn't support v2 (<= 1.0.1):
     use PyQt4 @v1
 
-Next, ask ETS' QT_API env variable
+Next, ask QT_API env variable
 
 if QT_API not set:
     ask matplotlib via rcParams['backend.qt4']
     if it said PyQt:
         use PyQt4 @v1
     elif it said PySide:
         use PySide
 
     else: (matplotlib said nothing)
         # this is the default path - nobody told us anything
         try:
             PyQt @v1
         except:
             fallback on PySide
 else:
-    use PyQt @v2 or PySide, depending on QT_API
-    because ETS doesn't work with PyQt @v1.
+    use what QT_API says
 
 """
+# NOTE: This is no longer an external, third-party module, and should be
+# considered part of IPython. For compatibility however, it is being kept in
+# IPython/external.
 
 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
+# TODO: Make sure this logic is still in sync with matplotlib's requirements.
+# In particular, matplotlib can also now support a qt5 backend, and so this will
+# break if matplotlib is imported and running happily with qt5, because
+# it only queries for the preferred qt4 option.
 def matplotlib_options(mpl):
     if mpl is None:
         return
     mpqt = mpl.rcParams.get('backend.qt4', None)
     if mpqt is None:
         return None
     if mpqt.lower() == 'pyside':
         return [QT_API_PYSIDE]
     elif mpqt.lower() == 'pyqt4':
         return [QT_API_PYQT_DEFAULT]
     raise ImportError("unhandled value for backend.qt4 from matplotlib: %r" %
                       mpqt)
 
 def get_options():
     """Return a list of acceptable QT APIs, in decreasing order of
     preference
     """
     #already imported Qt somewhere. Use that
     loaded = loaded_api()
     if loaded is not None:
         return [loaded]
 
     mpl = sys.modules.get('matplotlib', None)
 
     if mpl is not None and not check_version(mpl.__version__, '1.0.2'):
         #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