cpplint strictly requires Python 2

Using Python 3 (as is the default GitHub's 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

@@ -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 python3-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.

scripts/diff_to_added_lines.py

@@ -1,6 +1,5 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
-from __future__ import print_function
 import sys
 
 def eprint(*args, **kwargs):