cpplint strictly requires Python 2

Using Python 3 (as is the default in Ubuntu 20.04) requires merging
changes from https://github.com/cpplint/cpplint, which is a larger
undertaking.

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -352,7 +352,7 @@ jobs:
         run: ./.github/workflows/pull-request-check-clang-format.sh
 
   check-cpplint:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
         with:
@@ -364,7 +364,8 @@ jobs:
           # user input
           DEBIAN_FRONTEND: noninteractive
         run: |
-          pip install unidiff
+          sudo apt-get update
+          sudo apt-get install --no-install-recommends -yq python-unidiff
       - name: Check updated lines of code meet linting standards
         env:
           BASE_BRANCH: ${{ github.base_ref }}

scripts/cpplint.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 # -*- coding: utf-8 -*-
 #
 # Copyright (c) 2009 Google Inc. All rights reserved.