Streamlink supports plugin sideloading, if you go to the directory listed on that page (depending on your OS) and put the twitch.py file there, Streamlink should then use your custom file.
If a plugin is added with the same name as a built-in plugin, then the added plugin will take precedence. This is useful if you want to upgrade plugins independently of the Streamlink version.
Streamlink supports plugin sideloading, if you go to the directory listed on that page (depending on your OS) and put the twitch.py file there, Streamlink should then use your custom file.