fix: Use nightly SciPy wheels from Anaconda in HEAD of dependencies t…

…esting (#1533)

* Use nightly wheels of SciPy from Anaconda's PyPI to avoid building SciPy from source for 'HEAD of dependencies' workflow
   - Huge thanks to Ralf Gommers (@rgommers) for pointing this out!
   - c.f. https://anaconda.org/scipy-wheels-nightly/scipy
* Effectively reverts PR #1532

.github/workflows/dependencies-head.yml

@@ -23,13 +23,14 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
 
-    # For the time being only test SciPy pre-releases
-    # c.f. Issue #1531
+    # Use nightly SciPy wheels from Anaconda's PyPI
+    # c.f. https://twitter.com/ralfgommers/status/1419917265781334025
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip setuptools wheel
         python -m pip --no-cache-dir --quiet install --upgrade --editable .[test]
-        python -m pip install --upgrade --pre scipy
+        python -m pip uninstall --yes scipy
+        python -m pip install --upgrade --index-url https://pypi.anaconda.org/scipy-wheels-nightly/simple scipy
         python -m pip list
 
     - name: Test with pytest