fix gcd, coefficient and monomial_coefficient

[mantepse]

fix #40504

This is only patching the bugs. A more thorough redesign would be desirable, but is too much work for me right now.

In particular, I think that implementing __getattr__ to magically translate methods to the underlying polynomial ring is a bad idea.

[tscrim]

A few small things; otherwise LGTM.

[tscrim]

Can we be more specific about what type of exception this will raise (e.g., ValueError, TypeError?)

[mantepse]

I think this code is rather bad anyway, I only kept it to keep it consistently so.

If you want to, I would replace all the occurrences (not only in gcd) with

        try:
            result = self._p.gcd(x._p)
        except TypeError:
            ...  # compute result by first creating the correct parent
        return InfinitePolynomial_sparse(self.parent(), result)

[mantepse]

Actually, I just decided to factor out the logic that creates a common ring for the argument and self. This makes the code much more readable.

[github-actions]

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