Update matplotlib requirement from <3.8.0,>=3.0.0 to >=3.0.0,<3.9.0 in /_requirements (#273)

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_requirements/default.txt

@@ -1,5 +1,5 @@
 setuptools>=68.0.0, <68.2.0
-matplotlib>=3.0.0, <3.8.0
+matplotlib>=3.0.0, <3.9.0
 ipython[notebook]>=8.0.0, <8.15.0
 urllib3  # for ipython
 torch>=1.8.1, <2.1.0