use v2 action and pin python version (#4568)

.github/workflows/pre-commit.yml

     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - uses: actions/setup-python@v1
+    - uses: actions/setup-python@v2
+      with:
+        python-version: 3.7.x
     - uses: actions/setup-ruby@v1
       with:
         ruby-version: '2.6'