The semantics of `release_notes.py` was slightly confusing. It is meant
to be run a `script/release_notes.py v4.15.0` on the `releases/v4.16.0`
branch. To help, I've changed the usage to `script/release_notes.py
--since v4.15.0`.
This PR adds a script to automatically generate release notes using the
new `changelog-*` labels and "This PR ..." conventions.
Usage:
```
script/release_notes.py v4.X.0
```
where `v4.X.0` is the **previous** release, i.e. the script will process
all commits *since* that tag.