gh-37291: Support optional debugpy
Gentoo is removing it, but since only ipykernel has it as a dependency,
we can skip debugpy entirely when ipykernel from the system is
installed. This lets the system ipykernel package decide if it wants to
pull in debugpy.
URL: https://github.com/sagemath/sage/pull/37291
Reported by: Michael Orlitzky
Reviewer(s): Dima Pasechnik, Matthias Köppe, Michael Orlitzky
