avoid string version comparisons in external.qt

We have learned from pyzmq that one should *always* parse version strings before comparing them.

IPython/external/qt.py

@@ -7,7 +7,7 Do not use this if you need PyQt with the old QString/QVariant API.
 """
 
 import os
-
+from distutils.version import LooseVersion as V
 # Available APIs.
 QT_API_PYQT = 'pyqt'
 QT_API_PYSIDE = 'pyside'
@@ -26,7 +26,7 if QT_API is None:
     pyside_found = False
     try:
         import PySide
-        if PySide.__version__ < '1.0.3':
+        if V(PySide.__version__) < V('1.0.3'):
             # old PySide, fallback on PyQt
             raise ImportError
         # we can't import an incomplete pyside and pyqt4
@@ -49,7 +49,7 if QT_API is None:
                       "present.\nThis will likely crash, please install " \
                       "PySide completely, remove PySide or PyQt4 or set " \
                       "the QT_API environment variable to pyqt or pyside"
-            if QtCore.PYQT_VERSION_STR < '4.7':
+            if V(QtCore.PYQT_VERSION_STR) < V('4.7'):
                 # PyQt 4.6 has issues with null strings returning as None
                 raise ImportError
             QT_API = QT_API_PYQT
@@ -63,7 +63,7 elif QT_API == QT_API_PYQT:
 # Now peform the imports.
 if QT_API == QT_API_PYQT:
     from PyQt4 import QtCore, QtGui, QtSvg
-    if QtCore.PYQT_VERSION_STR < '4.7':
+    if V(QtCore.PYQT_VERSION_STR) < V('4.7'):
         raise ImportError("IPython requires PyQt4 >= 4.7, found %s"%QtCore.PYQT_VERSION_STR)
 
     # Alias PyQt-specific functions for PySide compatibility.
@@ -72,7 +72,7 if QT_API == QT_API_PYQT:
 
 elif QT_API == QT_API_PYSIDE:
     import PySide
-    if PySide.__version__ < '1.0.3':
+    if V(PySide.__version__) < V('1.0.3'):
         raise ImportError("IPython requires PySide >= 1.0.3, found %s"%PySide.__version__)
     from PySide import QtCore, QtGui, QtSvg