Use older numpy for older Python versions

The latest version refuses to run on anything older than Python 3.7.

.github/workflows/test.yml

@@ -26,6 +26,10 @@ jobs:
           python -m pip install --upgrade pip
           python -m pip install 'setuptools>=18.5,<=39.0.0'
           python -m pip install -r test-requirements.txt
+      - name: Install old numpy
+        run: |
+          python -m pip install 'numpy<1.17'
+        if: ${{ matrix.python-version >= '3.4' && matrix.python-version < '3.7' }}
       - name: Run tests
         run: |
           python setup.py nosetests --verbosity=3