fix the method monomials_of_degree

[DavidAyotte]

📚 Description

Fixes #35042.

The goal of this PR is to fix two bugs of the method monomials_of_degree. First, the code does not take into account the case of weighted polynomial ring:

sage: P = PolynomialRing(QQ, 3, 'x,y,z', order=TermOrder('wdeglex', [4,5,6]))
sage: P.inject_variables()
Defining x, y, z
sage: x.degree()
4
sage: P.monomials_of_degree(1)
[x, y, z]

Next, as pointed out by @videlec, the current list of monomial that is returned is not sorted:

sage: R.<x,y,z> = ZZ[]
sage: mons = R.monomials_of_degree(2)
sage: mons
[x^2, x*y, x*z, y^2, y*z, z^2]
sage: sorted(mons)
[z^2, y*z, x*z, y^2, x*y, x^2]
sage: 

CC: @videlec, @yyyyx4

📝 Checklist

• I have made sure that the title is self-explanatory and the description concisely explains the PR.
• I have linked an issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[yyyyx4]

Looks good so far, and sorry for not thinking of weighted polynomial rings when I wrote that method.

Could you add a doctest to demonstrate the corrected behavior?

[videlec]

Let me mention that there are two bugs in the initial implementation. The term ordering is also not taken care of in the output (it ought to be sorted).

[DavidAyotte]

Ok I added doctests. I'm not sure whether or not there is a better way of getting the degrees of the generators.

[videlec]

Could you add examples with different terms ordering (and no weighting)?

[videlec]

Nice!