Use int(characteristic) for PyPy

PyPy cannot handle 3-arg pow if the first two types don't know about the
third like pow(int, int, fmpz).

Author: oscarbenjamin

src/flint/test/test_all.py

@@ -2935,7 +2935,7 @@ def setbad(obj, i, val):
             assert x.pow_mod(4, x**2 + 1) == 1
 
             assert x.pow_mod(2**127, x - 1) == 1
-            assert (1 + x).pow_mod(2**127, x - 1) == pow(2, 2**127, characteristic)
+            assert (1 + x).pow_mod(2**127, x - 1) == pow(2, 2**127, int(characteristic))
 
             if type(x) is not flint.fq_default_poly:
                 assert (1 + x).pow_mod(2**127, x - 1, S(1)/2) == pow(2, 2**127, characteristic)