Workaround to prevent this calculation to blow the stack

Specifically, the CPython interpreter's symbol visibility analysis makes
recursive calls into a function called `symtable_visit_expr`, and can
overflow the stack when analyzing a very large expression of binary
operators (this also came up in this ticket:
https://trac.sagemath.org/ticket/16014).  While I seek a more general
fix to that problem, we need to work around it.  In this case the
polynomial `psi` is converted one term at a time, and then summed,
rather than summed first and then converted (resulting in evaluating a
very large Singular polynomial).  The result is the same either way
though might be slightly slower with this workaround, though I haven't
tested.

src/sage/schemes/elliptic_curves/isogeny_small_degree.py

@@ -1493,9 +1493,8 @@ def Psi2(l):
         t += [(c[n]-(4*n-2)*A*t[n-1]-(4*n-4)*B*t[n-2])*(1/QQ(4*n+2))]
     for n in range(1,d+1):
         s += [(-1/QQ(n))*sum((-1)**i*t[i]*s[n-i] for i in range(1,n+1))]
-    psi = sum((-1)**i*s[i]*x**(d-i) for i in range(0,d+1))
     R = PolynomialRing(QQ,['x','u','v'])
-    return R(psi)
+    return sum(R((-1)**i*s[i]*x**(d-i)) for i in range(0,d+1))
 
 
 def isogenies_prime_degree_genus_plus_0(E, l=None):