set 3.8 as minimum python version

Author: gasman

setup.py

@@ -13,6 +13,7 @@
     include_package_data=True,
     license='BSD',
     long_description=open('README.rst').read(),
+    python_requires=">=3.8",
     classifiers=[
         'Development Status :: 4 - Beta',
         'Environment :: Web Environment',