gh-40983: Ensure _charpoly_df is interruptible, take 2
previously I added some `sig_check` to this function in
https://github.com/sagemath/sage/pull/39757 , but I didn't realize this
loop does an O(n^2) amount of work without `sig_check`. (for p in
range(t+1), for k in range(p))
or maybe I did realize but didn't think O(n^2) is a big deal,
considering the whole algorithm is O(n^4).
@tobiasdiez report a test failure in
https://github.com/sagemath/sage/issues/40715#issuecomment-3369069147 .
Hopefully this fixes it, or at least reduce its probability.
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URL: https://github.com/sagemath/sage/pull/40983
Reported by: user202729
Reviewer(s): Travis Scrimshaw
