1. Store cython_debug in site-packages/sage/cython_debug instead of in the Cython build directory.

2. Upgrade cysignals:

Tarball: https://pypi.python.org/packages/92/15/9ca4a50304fca8debaccac2e9b630626c13f4c91992ca5a86f52732bb983/cysignals-1.6.9.tar.gz#md5=d8155169fefb8ea9c036c2a8ee1495bd

CC: @kiwifb

Component: build

Author: Jeroen Demeyer

Branch/Commit: fa789c2

Reviewer: François Bissey

Issue created by migration from https://trac.sagemath.org/ticket/21509