gnu: z3: Update to 4.8.3 and provide python3 bindings

* gnu/packages/maths.scm (z3): Update to 4.8.3.
[build-system]: Switch from cmake to make, and use the current
scripts/mk_make.py build script instead of the now-deprecated
contrib/cmake/bootstrap.py.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>