[proposal] Consider improving the Towncrier changelog structure

I've been experimenting with generic changelog improvements in some of my projects recently, and I ended up making the categories more granular. See https://yarl.aio-libs.org/en/latest/changes/#released-versions for examples of what it looks like (1.9.3 and newer).

@gaborbernat I want to propose that tox adopts a similar set of the fragment categories.