Add Release Drafter to draft releases

[hugovk]

I've not been updating CHANGES.rst for a while partly because it became too big/unwieldy to be editable on mobile. Also it's the sort of manual work that could and should be automated.

Let's automate it!

Release Drafter is a GitHub Action which runs when PRs are merged and I've been happily using it on other projects.

It takes the titles of PRs merged since the last release, and puts them into a draft release at https://github.com/python-pillow/pillow/releases

Drafts are only visible to maintainers, until they're published.

The idea is the draft will fill up, then on release day, the draft is published as usual.

If we still value putting the list into CHANGES.rst (do we?), we can copy and paste it during the release process (which includes updating CHANGES.rst anyway).

PRs labelled with changelog: skip are excluded.

If we want, we can further configure it to automatically put PRs into groups based on labels. Here's a good example:

• https://github.com/release-drafter/release-drafter/releases

I suggest running just with the basic config to begin, and consider categories later, although suggestions welcome!

The draft is updated each time the action runs, so any final touches should be made just before publishing. Or better: update the PR titles themselves, which is a good idea for future SEO.

[hugovk]

Going to merge this so we can see how it goes.

[hugovk]

Here's how the draft looks (putting in code format to avoid pinging people and issues):

https://pillow.readthedocs.io/en/stable/releasenotes/8.1.0.html

## Changes

- Add Release Drafter to draft releases #5058 [@hugovk]
- Update FriBiDi to 1.0.10 #5064 [@nulano]
- GHA: Install Raqm on macOS #5061 [@nulano]
- Updated libraqm to 0.7.1 #5063 [@radarhere]
- GHA: Make windows cache depend on VS version #5057 [@nulano]
- Docs: add logo with text to menu #5055 [@hugovk]
- Moved warning to end of execution #4965 [@radarhere]
- Add line breaks in test-windows #5052 [@nulano]
- Removed unused C methods #5026 [@radarhere]
- Updated libjpeg-turbo to 2.0.6 #5044 [@radarhere]
- Remove excess Travis CI jobs #5029 [@hugovk]
- Fixed link formatting #5040 [@radarhere]
- init() if one of the formats is unrecognised #5037 [@radarhere]
- PNG supports append_images #5031 [@radarhere]
- Link directly to pilfont script #5030 [@radarhere]
- Updated extra test images list #5027 [@radarhere]
- GHA: Add 3.10-dev #4980 [@nulano]
- Document FreeMono license #5002 [@radarhere]
- Refer to "HPND License" instead of "PIL Software License" #5025 [@radarhere]
- Added Fedora 33 to CI targets #5024 [@radarhere]
- Update Travis CI links for .com #5022 [@hugovk]
- Updated Python for diffcover and doccheck to 3.9 #5023 [@radarhere]
- Add Fedora 33 #5021 [@hugovk]
- Invoke pip using Python #5019 [@radarhere]
- Updated CI targets #5018 [@radarhere]
- AppVeyor: Test on Python 3.9 final #5014 [@hugovk]
- Describe effect of WebP save_all #5016 [@radarhere]
- Updated CI targets #5013 [@radarhere]
- Travis CI: Test on Python 3.9 final #5010 [@hugovk]
- Clarified that "text" must be a string #5007 [@radarhere]
- Releasing: Add a 'git push' to point release, adjust MD formatting #5006 [@hugovk]
- Updated macOS tested Pillow versions #5004 [@radarhere]
- CI: Replace test images from pillow-depends #5001 [@nulano]

[radarhere]

It turns out that including PRs since the last release becomes a problem when we have point releases - for example, 8.2.0 doesn't currently include #5258

[hugovk]

Ah yes, could be release-drafter/release-drafter#520 or release-drafter/release-drafter#756.