Are you using sessions? You could track the next parameter within the
session and after the password reset either add it as a hidden element
in the login form, or redirect the user back to the login page using a
URL that includes the ?next= value.
https://docs.djangoproject.com/en/1.8/topics/http/sessions/
Another option would be to keep the next parameter as part of the URL
all the way through the password reset process if you don't want to
store it in the session, but that would require that you grab it every
time the user loads a new page, which is probably more cumbersome than
sessions to generate the URL's and to capture/store the parameter upon
ever request.
-James