Work around inconsistent iteration in `multi_polynomial_sequence.py`

[S17A05]

When iterating over a pbori.BooleanPolynomial, the iterator generates just the monomials, whereas iterating over a MPolynomial_libsingular defined over GF(2) yields a tuple of the monomial together with its coefficient $1$. This causes issues in the method .coefficients_monomials() of PolynomialSequence_gf2; we fix said issue by checking whether the iteration yields tuples, in which case we simply replace the tuple by its second element (the actual monomial) in each iteration. which type of polynomial ring we are working in before iterating.

Fixes #37837 (but there might be a more elegant solution).

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[mantepse]

In case you are interested in the boolean polynomial ring, maybe you want to resurrect #26929.