LGTM

This merge may need to be partially reverted. xcpy module works within Topspin with Jython rather than CPython, which is why the antiquated style/features of Python 2.7 is used everywhere.

I just discovered that Jython only supports Python 2. Given the pace of change of Python libraries, I guess this means Jython is more or less dead. Any way, since the xcpy module works within TopSpin which includes Jython, there is no discussion here: I will revert xcpy.py and xcpy_test.py changes. Sorry about the mess.

BTW, I got confused by setup.py:

        'Programming Language :: Python :: 3',
        'Programming Language :: Python :: 3.6',
        'Programming Language :: Python :: 3.7',
        'Programming Language :: Python :: 3.8',
        'Programming Language :: Python :: 3.9',

Reverted in #183.