Add python_requires

See https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

Author: mgeier

setup.py

@@ -15,6 +15,7 @@
     py_modules=['jack'],
     setup_requires=['CFFI>=1.0'],
     install_requires=['CFFI>=1.0'],
+    python_requires='>=2.6',
     extras_require={'NumPy': ['NumPy']},
     cffi_modules=['jack_build.py:ffibuilder'],
     author='Matthias Geier',