Fetch numpy and contourpy from nightly for 3.13

They don't yet have wheels on PyPI for this Python version.

Author: QuLogic

.github/workflows/tests.yml

@@ -222,6 +222,13 @@ jobs:
             PRE="--pre"
           fi
 
+          # These don't have wheels on PyPI yet, but we can try the nightly wheels for now.
+          if [[ "${{ matrix.python-version }}" == '3.13' ]]; then
+            python -m pip install \
+              --index-url https://pypi.anaconda.org/scientific-python-nightly-wheels/simple \
+              --upgrade --only-binary=:all: contourpy numpy
+          fi
+
           # Install dependencies from PyPI.
           # Preinstall build requirements to enable no-build-isolation builds.
           python -m pip install --upgrade $PRE \