Use EXT_SUFFIX when available for determining Python library suffix

This is one of the patches that was required to build arc with PyPy for conda-forge. Basically replaces .${PYTHON_SOABI}.so with ${EXT_SUFFIX} for the _arc Python module.

I've added 84502407 as I guess you're still supporting Python 2. If arc6 is Python 3 only I'll revert it.

See https://www.python.org/dev/peps/pep-3149/ for more details about EXT_SUFFIX.

Note: merged this one by hand as I forgot to add copy_mr:next label.