[CI] Enable CI for all PRs, fix yapf to 0.31.0, flake8 to 3.9.2 (#235)

.github/workflows/ci.yml

@@ -1,12 +1,11 @@
 name: CI
 
-# Controls when the action will run. Triggers the workflow on push or pull request
+# Controls when the action will run. Triggers the workflow on push
 # events but only for the master branch
 on:
   push:
     branches: [ master ]
   pull_request:
-    branches: [ master ]
 
 jobs:
   build-python:
@@ -29,7 +28,7 @@ jobs:
       - name: Install Python dependencies
         run: |
           python -m pip install -r tests/requirements.txt
-          python -m pip install yapf flake8
+          python -m pip install yapf==0.31.0 flake8==3.9.2
 
       - name: Check Python formatting
         run: |