Enable testing against nightly upstream wheels

[lagru]

Description

See https://github.com/scientific-python/upload-nightly-action#using-nightly-builds-in-ci

Somewhat related #6514.

cc @bsipocz, @jarrodmillman

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8
• Descriptive commit messages (see below)

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.
• There is a bot to help automate backporting a PR to an older branch. For
  example, to backport to v0.19.x after merging, add the following in a PR
  comment: @meeseeksdev backport to v0.19.x
• To run benchmarks on a PR, add the run-benchmark label. To rerun, the label
  can be removed and then added again. The benchmark output can be checked in
  the "Actions" tab.

[bsipocz]

Looks all good, thanks for doing these updates.

Also, add a development, or the magic "fix" word that this closes #6956

[lagru]

Also, add a development, or the magic "fix" word that this closes #6956

Not sure what you mean? "#6956" is this PR?

[lagru]

Also, it was pointed out to me, this solution using --extra-index-url https://pypi.anaconda.org/scientific-python-nightly-wheels/simple isn't expected to include all nightly wheels right now. But it's expected upstream projects like NumPy will start providing wheels there. You can see under https://pypi.anaconda.org/scientific-python-nightly-wheels/simple/ which nightly wheels are already available...

[lagru]

One thing I'm not sure about is whether we want to test nightly wheels on every pull request. I don't want contributors to get confused and frustrated because they are assuming it's something they did. So we could turn this into a regular actions that's only triggered for main like networkx did, but others pointed out that then there are no automatic notifications for everyone if there is a failure...

[lagru]

@jarrodmillman, this should be ready to merge now.