Run CI on numpy<2.1 for now

.github/workflows/python-package.yml

@@ -5,7 +5,7 @@ name: Python package
 
 on:
   push:
-    branches: [ master ]
+    branches: [ master, test-github-action ]
   pull_request:
     branches: [ master ]
 
@@ -28,6 +28,7 @@ jobs:
           python -m pip install --upgrade pip
           python -m venv env
           source env/bin/activate
+          pip install numpy<2.1  # temporary solution for apparently broken numpy release candidate
           pip install setuptools
           pip install cython flake8 pytest
           python setup.py install