Numpy deprecated asscalar in 1.16. This will be fixed in the next
matplotlib version, but to avoid deprecation warnings (and because they
are breaking the sage testsuite), it can't hurt to backport the fix
already.
Upstream: https://github.com/matplotlib/matplotlib/pull/12478
We currently do not build mathplotlib with any backend. This can be very
confusing for users. They will try to use matplotlib and it will simply
display nothing (see #51337). We should ship at least one backend. `tk`
was chosen somewhat arbitrarily. The gtk backend is problematic (see
https://github.com/NixOS/nixpkgs/issues/50959#issuecomment-441327767)
so tkinter seems like a good choice.
There is already a backend provided on darwin so there is no reason to
include tk there.
