[skip ci] fix SyntaxWarning: 'is not' with a literal. Did you mean '!='?

src/Mod/PartDesign/WizardShaft/Shaft.py

@@ -88,7 +88,7 @@ def addSegment(self, l, d, di):
         # We don't call equilibrium() here because the new segment has no constraints defined yet
         # Fix face reference of fixed segment if it is the last one
         for i in range(1,  len(self.segments)):
-            if self.segments[i].constraintType is not "Fixed":
+            if self.segments[i].constraintType != "Fixed":
                 continue
             if i == len(self.segments) - 1:
                 self.segments[index].constraint.References = [( self.feature.feature,  "Face%u" % (2 * (index+1) + 1) )]