The naming of the variables is definitely confusing here. A 'short' summary output was actually always sent to the `summary_stream`, where the full list of deprecations was send to the `deprecation_stream`. Since we defer printing any deprecations at all until the summary, printing to the `deprecation_stream` is triggered by the `deprecation_summary`.

To make this more clear, maybe I should extract a method named `print_deferred_deprecation_warnings` ? Would that make more sense why it's printing to that stream?