matplotlib: support 3.6
The bitmap
backend is gone and the agg
backend was its baseclass.
Use it directly. Note that the return type changes from a 2-tuple to a
6-tuple and the image is now index 5.
Tested back to 3.1. Documented in 2.0.0 as well.
Fixes: #18517 (closed)
Backport: release
Backport: paraview/release