fix test for webkit

Was getting an import error when trying to import the webengine version.
Not going to skip the tests using this step since they seemed to be
working otherwise, probably this behavior isn't needed on webkit anyway.

tests/end2end/features/test_prompts_bdd.py

@@ -8,7 +8,10 @@ import pytest_bdd as bdd
 bdd.scenarios('prompts.feature')
 
 from qutebrowser.utils import qtutils
-from qutebrowser.qt.webenginecore import PYQT_WEBENGINE_VERSION
+try:
+    from qutebrowser.qt.webenginecore import PYQT_WEBENGINE_VERSION
+except ImportError:
+    PYQT_WEBENGINE_VERSION = None
 
 
 @bdd.when("I load an SSL page")
@@ -48,6 +51,7 @@ def fresh_instance(quteproc):
     # browser to make it forget previous prompts.
     if (
         qtutils.version_check("6.8", compiled=False)
+        and PYQT_WEBENGINE_VERSION
         and PYQT_WEBENGINE_VERSION < 0x60800
     ):
         quteproc.terminate()