gh-37605: Ensure returned degree of multivariate polynomial is type `Integer` for `MPolynomialRing_libsingular` class
Previously the degree of multivariate polynomials was returned as a
python `int` instead of a SageMath `Integer` resulting in ugly
`ZZ(f.degree())` calls in various places.
This PR only focuses on the case of `MPolynomialRing_libsingular` to
keep the noise of the PR as low as possible.
**Future work**: This same issue is in `MPolynomial_polydict`, I am
happy to also do the work here, or make a new PR -- really I don't like
sage returning a `int` when we dont have to.
**Edit**
This follow up work has been done in PR:
https://github.com/sagemath/sage/pull/37611
URL: https://github.com/sagemath/sage/pull/37605
Reported by: Giacomo Pope
Reviewer(s): Matthias Köppe
