Define minimal Python version in setup.py

Closes #474
Closes #475
Fixes 3542c06

Author: cedk

setup.py

@@ -75,6 +75,7 @@
         'Topic :: Text Processing :: General',
     ],
     packages=find_packages(),
+    python_requires='>=3.8',
     install_requires=[],
     package_data={'': ['*.dat', '*.crt', 'py.typed']},
     extras_require={