Add matplotlib test dependency so we can run notebook_utils_test.

PiperOrigin-RevId: 454823630
Change-Id: I9903ca5eb2284c8775631e2ffaf452869c7d4d7c

setup.py

@@ -42,7 +42,10 @@ setup(
         'scipy',
         'tensorflow-cpu',
     ],
-    tests_require=['mock'],
+    tests_require=[
+        'matplotlib',  # For notebook_utils_test.
+        'mock',
+    ],
     classifiers=[
         'Development Status :: 5 - Production/Stable',
         'Intended Audience :: Science/Research',