fix pip install numpy version

Author: benbovy

.github/workflows/tests.yml

@@ -104,5 +104,5 @@ jobs:
 
       - name: Run tests (Numpy 1.xx)
         run: |
-          python -m pip install numpy==1 pytest
+          python -m pip install numpy==1.26.4 pytest
           pytest python/fastscapelib/tests -vv --color=yes