build(deps-dev): update matplotlib requirement

Updates the requirements on [matplotlib](https://github.com/matplotlib/matplotlib) to permit the latest version.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.5.3...v3.8.0)

---
updated-dependencies:
- dependency-name: matplotlib
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

pyproject.toml

@@ -95,7 +95,7 @@ testing = [
     "ipython!=8.1.0,<8.16",
     "ipywidgets>=8",
     "jupytext>=1.11.2,<1.15.0",
-    "matplotlib>=3.5.3,<3.6",  # TODO mpl 3.6 subtly changes output of test regressions
+    "matplotlib>=3.5.3,<3.9",  # TODO mpl 3.6 subtly changes output of test regressions
     "nbdime",
     "numpy",
     "pandas",