Improve CI

- Run on pull requests
- Add Python 3.10 to test matrix
- Upgrade to `actions/checkout@v2`

.github/workflows/check.yml

@@ -1,17 +1,21 @@
 name: Python package
 
-on: [push]
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
 
 jobs:
   build:
     runs-on: ubuntu-latest
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.6, 3.7, 3.8, 3.9, '3.10']
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2
       with: