CHORE: Use matplotlib Agg backend when testing #336

Job	Run time
Check the title of the pull request	5s
Documentation style check	30s
Build wheelhouse and smoke tests (ubuntu-latest, 3.7)	1m 58s
Build wheelhouse and smoke tests (ubuntu-latest, 3.8)	2m 8s
Build wheelhouse and smoke tests (ubuntu-latest, 3.9)	2m 26s
Build wheelhouse and smoke tests (ubuntu-latest, 3.10)	2m 31s
Build wheelhouse and smoke tests (ubuntu-latest, 3.11)	2m 5s
Build wheelhouse and smoke tests (windows-latest, 3.7)	3m 26s
Build wheelhouse and smoke tests (windows-latest, 3.8)	4m 2s
Build wheelhouse and smoke tests (windows-latest, 3.9)	3m 38s
Build wheelhouse and smoke tests (windows-latest, 3.10)	3m 43s
Build wheelhouse and smoke tests (windows-latest, 3.11)	3m 40s
Documentation build without examples	17m 52s
Documentation build with examples	0s
Testing solvers and coverage (Linux)	43m 30s
Testing and coverage (Windows)	52m 13s
Testing and coverage (Linux)	1h 13m 38s
Testing IronPython and coverage (Windows)	1m 26s
Testing solvers and coverage (Windows)	30m 34s
Package library	0s
Release project	0s
Upload release documentation	0s
Deploy stable docs index	0s
	4h 9m 25s

Provide feedback