Upload wheels to GitHub Releases

[hugovk]

Fixes #146, using one of @mattip's suggestions.

Based on Cython's config:

• https://github.com/MacPython/cython-wheels/blob/b421040fd59ccd1cee9b2cb66b79a3d6bacde444/.travis.yml#L116-L125

Travis CI docs:

• https://docs.travis-ci.com/user/deployment/releases/

The wheels are only built for tags, and the workflow will change a bit so we'll need to update:

• the main RELEASING.md checklist,
• this README.md
• and update-pillow-tag.sh.

When releasing, we add and push a tag. It builds as normal (and also skips the "latest" jobs, we don't need to build/deploy them). The wheels are uploaded to a matching tag release here in this repo.

We can then download the wheels from the release, and can twine upload with the separately-built Windows wheels, and source (this looks handy for downloading).

(We could delete the release/files from here if we wanted to ensure PyPI is the place to go, not sure if that's necessary.)

Here's an example:

diff --git a/update-pillow-tag.sh b/update-pillow-tag.sh
index a48d6ca..c3000f1 100755
--- a/update-pillow-tag.sh
+++ b/update-pillow-tag.sh
@@ -15,4 +15,5 @@ git fetch --all
 git checkout $1
 cd ..
 git commit -m "Pillow -> $1" Pillow
-git push
+git tag "$1"
+git push --tags

Note we'd still need a normal git push (without --tags), but that will do a normal build of latest and non-latest (and no wheels), so that could come in the post-release thing, to avoid eating up CI time.

I'll annotate some lines too with some extra info.

[hugovk]

Not needed, no longer using wheelhouse_uploader.

[hugovk]

Prevent the latest jobs building for tag builds.

[hugovk]

For info to confirm a wheel was built.

[hugovk]

This is an encrypted OAuth token created using travis setup releases on my account when inside a local checkout of the https://github.com/python-pillow/pillow-wheels/ repo (ie. not my fork).

https://docs.travis-ci.com/user/deployment/releases/#authenticating-with-an-oauth-token

[hugovk]

So we can use *.whl in the next line.

[hugovk]

Redundant with if: tag IS blank, but a useful extra guard to make sure we don't build wheels for latest.

[hugovk]

This makes sure the deploy is only run from this repo.

[hugovk]

And only for tags.

[hugovk]

Finally, tell Travis not to delete all the files, including wheels, before deploy.

https://docs.travis-ci.com/user/deployment#uploading-files-and-skip_cleanup

[hugovk]

And here's a test build on my fork (ignore the release is 7.2.1 and the files are 7.2.0, was just for test):

• https://github.com/hugovk/pillow-wheels/releases/tag/7.2.1
• https://travis-ci.org/github/hugovk/pillow-wheels/builds/705787072