There is also a migration guide here
https://dev.twitch.tv/docs/v5/guides/migration/
That may help or speed your migration
As dist says, all you should need to do is swap the header to be v5 instead of v3 (or add a header if you were not sending one in the first place)
v3 is the default API version and developers will need to set an Accept header to specify that they want to reach v5 endpoints. Developers can proactively include the v5 Accept header in order to properly test their updates. Below is an example and you can visit the Requests documentation for more information.
Is the key point from the Migration Guide.
For the URL you are calling /kraken/user you just need a new header. And then handle any keys that are not in the v5 output, but login/display_name are unchanged.
For session validation you may want to look at the “validation” endpoint instead
Which is neither a kraken (v3/v5) or helix endpoint