FIX: set appropriate QT_API, defaulting to pyqt5 with pyside2 as fall-back

Author: bergtholdt

ipykernel/eventloops.py

@@ -126,7 +126,15 @@ def loop_qt4(kernel):
 def loop_qt5(kernel):
     """Start a kernel with PyQt5 event loop integration."""
     if os.environ.get('QT_API', None) is None:
-        os.environ['QT_API'] = 'pyqt5'
+        try:
+            import PyQt5
+            os.environ['QT_API'] = 'pyqt5'
+        except ImportError:
+            try:
+                import PySide2
+                os.environ['QT_API'] = 'pyside2'
+            except ImportError:
+                pass
     return loop_qt4(kernel)