prevent segfaults with flint 2.6

this allows running Sage with system Flint 2.6.x - carved out of #29719

It does not fix all the doctests - but these
remaining fixes are on #29719

src/sage/schemes/hyperelliptic_curves/monsky_washnitzer.py

@@ -1876,9 +1876,9 @@ def __init__(self, Q, R=None, invert_y=True):
             if not hasattr(self, '_curve'):
                 if self._Q.degree() == 3:
                     ainvs = [0, self._Q[2], 0, self._Q[1], self._Q[0]]
-                    self._curve = EllipticCurve(ainvs)
+                    self._curve = EllipticCurve(ainvs, check_squarefree=R.is_field())
                 else:
-                    self._curve = HyperellipticCurve(self._Q)
+                    self._curve = HyperellipticCurve(self._Q, check_squarefree=R.is_field())
 
         else:
             raise NotImplementedError("Must be an elliptic curve or polynomial "