Add CI job for Python2 compat checks

.github/workflows/ci.yml

@@ -14,6 +14,16 @@ jobs:
     - name: Check format
       run: ./check-fmt
 
+  py2-compat:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: cachix/install-nix-action@v16
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: actions/checkout@v2.4.0
+    - name: Check format
+      run: nix-shell --run 'git ls-files | grep -P "\.py$" | xargs py2-astparse'
+
   matrix_generate:
     runs-on: ubuntu-latest
     outputs: