* gnu/packages/python-xyz.scm (python-matplotlib): Update to 3.1.1.
[native-inputs]: Remove python-nose; add python-pytest and unzip; add
jquery-ui.
[arguments]: Add build phases "fix-and-disable-failing-tests" and
"install-jquery-ui", and move the "check" phase after the "install" phase;
use the default backend in setup.cfg.
(python2-matplotlib): Keep at version 2.2.3.
(python-matplotlib-documentation)[arguments]: Remove "check" phase.