Add test that checks for regression in 3.9.1

This test checks for the fix made in 5803d1.

gaphas/solver/solver.py

@@ -133,6 +133,11 @@ def request_resolve_constraint(self, c: Constraint) -> None:
         elif self._marked_cons.count(c) < self._resolve_limit:
             self._marked_cons.append(c)
 
+    @property
+    def needs_solving(self) -> bool:
+        """Return if there are constraints that need solving."""
+        return bool(self._marked_cons)
+
     def solve(self) -> None:  # sourcery skip: while-to-for
         """
         Example:
@@ -164,6 +169,7 @@ def solve(self) -> None:  # sourcery skip: while-to-for
         >>> c._value
         10.0
         """
+        # NB. marked_cons is updated during the solving process
         marked_cons = self._marked_cons
         notify = self._notify
         try:

tests/test_solver.py

@@ -141,3 +141,22 @@ def handler(constraint):
     solver.solve()
 
     assert outer in events
+
+
+def test_needs_solving():
+    solver = Solver()
+    a = Variable()
+    b = Variable()
+    eq = EqualsConstraint(a, b)
+
+    solver.add_constraint(eq)
+    assert solver.needs_solving
+
+    solver.solve()
+    assert not solver.needs_solving
+
+    a.value = 3
+    assert solver.needs_solving
+
+    solver.solve()
+    assert not solver.needs_solving