In setup.py, the MACOS_DEPLOYMENT_TARGET is determined by the following code in setup.py:

    """ Prepares the environment for OS X building """
    darwin_major_to_osx_map = {
        '11': '10.7',
        '12': '10.8',
        '13': '10.9',
        '14': '10.10',
        '15': '10.11',
        '16': '10.12',
        '17': '10.13',
        '18': '10.14',
        '19': '10.15',
    }

This table is outdated, (and this will be a recurring problem). My darwin major for OSX 11.5 is "20". I'm not really sure
what deployment target I should set: 11.5?

Not setting the deployment target leads to the dreaded "locale not found" error. Adding a 20 to the table (pointing to 11.5) eliminates that problem.

p.s. for @hainm: similar problems arise for pytraj: I haven't yet investigated those....