fix pyqt6 pyside6 tests

tox.ini

@@ -1,6 +1,6 @@
 # For more information about tox, see https://tox.readthedocs.io/en/latest/
 [tox]
-envlist = py{37,38,39}-{linux,macos,windows}-{pyqt5,pyside2}
+envlist = py{37,38,39}-{linux,macos,windows}-{pyqt5,pyside2,pyqt6,pyside6}
 toxworkdir=/tmp/.tox
 
 [gh-actions]
@@ -22,6 +22,8 @@ PLATFORM =
 BACKEND =
     pyqt5: pyqt5
     pyside2: pyside2
+    pyqt6: pyqt6
+    pyside6: pyside6
 
 [testenv]
 platform =
@@ -35,4 +37,6 @@ extras =
     testing
     pyqt5: pyqt5
     pyside2: pyside2
+    pyqt6: pyqt6
+    pyside6: pyside6
 commands = pytest -v --color=yes --cov=pyqrangeslider --cov-report=xml