the weewx-windy extension is working as of v0.2, with either python2 or python3.
we tried using POST, since that is the 'right' way to do http uploads. unfortunately, despite what their web pages say, windy.com does not seem to support POST, at least not as of 18apr2019.
there were some hiccups using GET. the windy.com servers will accept only imperial units, not metric units, even though the server responds with a 'Success' when you post metric units. once again, this is as of 18apr2019.
hopefully windy.com will fix their api and/or documentation.
m