What's causing 'unable to connect to data source' for pyodbc?

What's causing 'unable to connect to data source' for pyodbc?

After hours of going in circles it turns out all I was missing was

TDS_Version = 8.0 in the DSN in my odbc.ini file.

I had specified it elsewhere, but it needed to be here, too, apparently.

Hope this helps some other poor soul.