Make generic polynomial multiplication interruptible

[user202729]

As in the title.

Benchmark can be done as follows

sage: from sage.doctest.util import ensure_interruptible_after
sage: R.<x> = CDF[]
sage: f = R.random_element(degree=5000)
sage: g = R.random_element(degree=5000)
sage: %time h = f*f
sage: %time h = f*g

The first one takes ≈ 2.32s and the second one takes ≈ 2.9s on my machine. Additional slowdown is within measurement error anyway (certainly checking a single bool is faster than constructing a new Python object).

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