ci: Use scientific-python-nightly-wheels for dependency nightlies (#430)

* Nightly wheels for the Scientific Python community have migrated from
  the Anaconda Cloud https://anaconda.org/scipy-wheels-nightly/ package
  index to https://anaconda.org/scientific-python-nightly-wheels.
   - c.f. https://scientific-python.org/specs/spec-0004/
   - c.f. scientific-python/specs#182
   - c.f. matplotlib/matplotlib#25950
* For matplotlib use --index-url of scientific-python-nightly-wheels and
  use --extra-index-url of public PyPI (https://pypi.org/) as some of
  matplotlib's dependencies aren't on scientific-python-nightly-wheels.

.github/workflows/head-dependencies.yml

@@ -76,7 +76,12 @@ jobs:
         python -m pip install --upgrade pip setuptools wheel
         python -m pip install pytest-github-actions-annotate-failures
         python -m pip --no-cache-dir --quiet install --upgrade .[test]
-        python -m pip install --pre --upgrade --extra-index-url https://pypi.anaconda.org/scipy-wheels-nightly/simple matplotlib
+        python -m pip install \
+          --upgrade \
+          --pre \
+          --index-url https://pypi.anaconda.org/scientific-python-nightly-wheels/simple \
+          --extra-index-url https://pypi.org/simple/ \
+          matplotlib
 
     - name: List installed Python packages
       run: python -m pip list