gh-37094: Fix bug with pari interface in `classical_modular_polynomial`
There is a bug in the new `classical_modular_polynomial` function
created in https://github.com/sagemath/sage/pull/36190.
A simple example to reproduce the bug is the following
```
p = 2^216 * 3^137 - 1
F.<i> = GF(p^2, modulus=[1,0,1])
E = EllipticCurve(F, [0, 6, 0, 1, 0])
classical_modular_polynomial(2, E.j_invariant())
```
This will fail with a `TypeError`.
The bug is due to the interface with the pari function `polmodular`. In
particular, contrary to the [documentation](http://pari.math.u-
bordeaux.fr/dochtml/ref/Polynomials_and_power_series.html#polmodular),
the `polmodular` function will only evaluate $\Phi_\ell(j, Y)$ for
$j$-invariants that are defined over $\mathbb F_p$, and not over any
generic finite field.
If however `j.parent()` is a generic finite field, but `j` itself is
defined over the prime field, pari will evaluate that and the current
implementation fails to convert back to a sage polynomial.
The proposed fix is to cast the result of the pari call to `ZZ['Y']`,
since the result of `polmodular` currently returns a polynomial in the
correct $\mathbb Z/n\mathbb Z[Y]$.
#sd123
URL: https://github.com/sagemath/sage/pull/37094
Reported by: Riccardo Zanotto
Reviewer(s): Frédéric Chapoton, Peter Bruin, Riccardo Zanotto
