Bug#1074627: gammapy: tests fail with matplotlib 3.8

[Drew Parsons]

Source: gammapy
Followup-For: Bug #1074627
Control: forwarded 1074627 https://github.com/gammapy/gammapy/issues/4754
Control: tags 1074627 fixed-upstream

Upstream patch https://github.com/gammapy/gammapy/pull/4755