Fix constraint union treating "not in" same as "!=" (#925)

Author: dimbleby

src/poetry/core/constraints/generic/constraint.py

@@ -180,7 +180,7 @@ def union(self, other: BaseConstraint) -> BaseConstraint:
 
             ops = {self.operator, other.operator}
             if (
-                (ops in ({"!="}, {"not in"}))
+                ops == {"!="}
                 or (
                     (
                         ops in ({"in", "!="}, {"in", "not in"})

tests/constraints/generic/test_constraint.py

@@ -1203,7 +1203,15 @@ def test_intersect_extra(
         (
             Constraint("tegra", "not in"),
             Constraint("rpi", "not in"),
-            AnyConstraint(),
+            # NB not AnyConstraint - eg "tegra-rpi" does not satisfy either constraint
+            (
+                UnionConstraint(
+                    Constraint("tegra", "not in"), Constraint("rpi", "not in")
+                ),
+                UnionConstraint(
+                    Constraint("rpi", "not in"), Constraint("tegra", "not in")
+                ),
+            ),
         ),
         (
             Constraint("tegra", "in"),