You need to use page parameter, e.g. for next 30 page = 2
https://api.github.com/search/issues?q=stress+test+label:bug+language:python+state:closed&page=2
You can also use per_page parameter to change the default size of 30. It supports max size of 100. Like this:
https://api.github.com/search/issues?q=stress+test+label:bug+language:python+state:closed&per_page=100
More detail can be found here