Merge pull request #1203 from ocefpaf/python_requires

add python_requires

Author: jswhit

setup.py

@@ -742,6 +742,7 @@ def _populate_hdf5_info(dirstosearch, inc_dirs, libs, lib_dirs):
       package_dir={'':'src'},
       package_data={"netCDF4.plugins": ["lib__nc*"]},
       ext_modules=ext_modules,
+      python_requires=">=3.6",
       **setuptools_extra_kwargs)
 
 # remove plugin files copied from outside source tree