[MNT] Update nightly wheels install location

I had originally thought this was included in matplotlib#25950 (hence why I had waited so long to merge it...), but apparently it was not.

Now that both pandas and numpy are uploading to the new location, this is the expected install location for new nightlies

Author: ksunden

.github/workflows/tests.yml

@@ -259,7 +259,7 @@ jobs:
         run: |
           python -m pip install pytz  # Must be installed for Pandas.
           python -m pip install \
-            --index-url https://pypi.anaconda.org/scipy-wheels-nightly/simple \
+            --index-url https://pypi.anaconda.org/scientific-python-nightly-wheels/simple \
             --upgrade --only-binary=:all: numpy pandas
 
       - name: Install Matplotlib