setup.py: replace broad python3 classifier with versions the pipeline tests against

setup.py

@@ -17,6 +17,11 @@ setuptools.setup(
     classifiers=[
         "License :: OSI Approved :: MIT License",
         "Operating System :: POSIX :: Linux",
-        "Programming Language :: Python :: 3",
+        # .github/workflows/python.yml
+        "Programming Language :: Python :: 3.5",
+        "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.8",
+        "Topic :: Utilities",
     ],
 )