"Except" did not match properly

Making the testGrammer test case strict showed that "except" was not
matched properly. Fixed by using onOf i.s.o. | operator

cadquery/selectors.py

@@ -569,7 +569,7 @@ def _makeExpressionGrammar(atom):
     #define operators
     and_op = Literal('and')
     or_op =  Literal('or')
-    delta_op =  Literal('exc') | Literal('except')
+    delta_op = oneOf(['exc','except'])
     not_op = Literal('not')
 
     def atom_callback(res):

tests/TestCQSelectors.py

@@ -453,6 +453,6 @@ class TestCQSelectors(BaseTest):
                        'bottom',
                        'not |(1,1,0) and >(0,0,1) or XY except >(1,1,1)[-1]',
                        '(not |(1,1,0) and >(0,0,1)) exc XY and (Z or X)']
-        
-        for e in expressions: gram.parseString(e)
+        #import pdb; pdb.set_trace()
+        for e in expressions: gram.parseString(e,parseAll=True)