TST/MAINT: Add oldest-supported-numpy

This uses the SciPy package `oldest-supported-numpy` to test
against the oldest supported version of NumPy for that specific
architecture. This is useful to support old features.

Author: Kai-Striega

.github/workflows/pythonpackage.yml

@@ -9,7 +9,8 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", 'pypy3.7', 'pypy3.8', 'pypy3.9']
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11"]
+        numpy-version: ["oldest-supported-numpy", "numpy"]
 
     steps:
     - uses: actions/checkout@v3
@@ -20,7 +21,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install numpy
+        pip install ${{ matrix.numpy-version }}
     - name: Lint with flake8
       run: |
         pip install flake8