user_config.authenticate was a Pywikibot-compat feature, recently re-implemented in core T89417. With the switch to requests, this has been unimplemented again.
The fallback is to use netrc to specify the basic auth credentials instead of user_config.
http://docs.python-requests.org/en/latest/user/authentication/#netrc-authentication
I am not sure if/how that works on Windows.