Specify python requirement

Author: HaarigerHarald

setup.py

@@ -83,6 +83,7 @@ def build_extension(self, ext):
     long_description_content_type="text/markdown",
     url="https://github.com/HaarigerHarald/geant4_pybind",
     license="Public Domain",
+    python_requires='>=3.5, <4',
     ext_modules=[CMakeExtension("geant4_pybind")],
     cmdclass={"build_ext": CMakeBuild},
     zip_safe=False