python.pkgs.matplotlib: add numpy 1.6 compat patch (#55577)

* python.pkgs.matplotlib: add numpy 1.6 compat patch

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

* sage: add numpy 1.16 compatibility patch

https://trac.sagemath.org/ticket/27000