Not working with Python 3.8

Hi,
this tool doesn't seem to work with recent versions of Python 3.

```
# mssql-cli
Traceback (most recent call last):
File "/usr/lib/python3.8/runpy.py", line 194, in _run_module_as_main
return _run_code(code, main_globals, None,
File "/usr/lib/python3.8/runpy.py", line 87, in _run_code
exec(code, run_globals)
File "/usr/local/lib/python3.8/dist-packages/mssqlcli/main.py", line 11, in
from mssqlcli.config import config_location
ImportError: cannot import name 'config_location' from 'mssqlcli.config' (/usr/local/lib/python3.8/dist-packages/mssqlcli/config.py)
```

Thanks!