Fix univariate polynomial reverse

[vneiger]

For univariate polynomials over "non-prime" finite fields, the reverse method had an issue when specifying degree=0: it raised an error saying that this optional argument should be... nonnegative (which 0 is).

Example:

sage: ring.<x> = GF(9)[]
sage: pol = ring(1)
sage: pol.reverse(degree=2)
x^2
sage: pol.reverse()
1
sage: pol.reverse(degree=0)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[5], line 1
----> 1 pol.reverse(degree=Integer(0))

File ~/repositories/software/sage/src/sage/rings/polynomial/polynomial_zz_pex.pyx:537, in sage.rings.polynomial.polynomial_zz_pex.Polynomial_ZZ_pEX.reverse()
    535 if degree is not None:
    536     if degree <= 0:
--> 537         raise ValueError("degree argument must be a non-negative integer, got %s" % (degree))
    538     try:
    539         d = degree

ValueError: degree argument must be a non-negative integer, got 0

This is fixed in this PR, along with fixes in similar code where the error message was not the expected one ("cannot convert to long", instead of "degree must be nonnegative").

[xcaruso]

Looks good to me.

[vneiger]

Minor fix: the change of the error message (upon negative input value) affects a doctest in matrix_polynomial_dense, this is fixed.

[github-actions]

Documentation preview for this PR (built with commit 9f15f69; changes) is ready! 🎉