Most of the time the answer is "absolutely nothing". From framework (and framework user) point of view there is no difference which protocol version is used, request handling is exactly the same. Protocols are handled somewhere else (by nginx/uwsgi/whatever). Http2 does not provide replacement for websockets.

There are some features and tricks you can use, and some optimizations which are done differently, but this also mostly is either outside of framework scope, or so use-case specific that no common patterns have been accepted widely enough to reach web framework.