Update to the latest matplotlib, currently version 3.2.1. This is Python 3-only, so it will be for Sage 9.2. This is necessary for upgrading Sphinx (#28856).

File: https://files.pythonhosted.org/packages/4a/30/eb8e7dd8e3609f05c6920fa82f189302c832e5a0f6667aa96f952056bc0c/matplotlib-3.2.1.tar.gz

CC: @mkoeppe @egourgoulhon

Component: packages: standard

Author: John Palmieri, François Bissey

Branch/Commit: eee7843

Reviewer: Dave Morris, Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/29547