cap matplotlib version for compat with old Python

pyproject.toml

@@ -54,7 +54,7 @@ dependencies = [
     "scipy>=1.5.4",
     "scikit-image>=0.17.2",
     "tifffile>=2020.8.25",
-    "matplotlib>=3.3.4",
+    "matplotlib>=3.3.4,<3.6",
     "SimpleITK>=2.0.2",
     "progressbar>=2.5",
     "python-decouple>=3.4",