Add numpy

Author: dongjoon-hyun

.github/workflows/master.yml

@@ -52,7 +52,7 @@ jobs:
       run: ./dev/lint-java
     - name: Python
       run: |
-        pip install flake8 sphinx
+        pip install flake8 sphinx numpy
         ./dev/lint-python
     - name: License
       run: ./dev/check-license