Set python-requires to >=3.7

Author: vrslev

setup.py

@@ -65,7 +65,7 @@ def get_tag(self):
     packages=packages,
     package_data=package_data,
     zip_safe=zip_safe,
-    python_requires='>=3',
+    python_requires='>=3.7',
     setup_requires=['CFFI>=1.0'],
     install_requires=['CFFI>=1.0'],
     extras_require={'NumPy': ['NumPy']},