strip python2-only coding:utf-8 & print_function settings

We're committed to Python 3 at this point, so clean up boilerplate.

Bug: https://crbug.com/gerrit/10418
Change-Id: Ib1719ba2eb65c53b94881a1a1bf203ddfcaaafed
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/292382
Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

run_tests

@@ -1,5 +1,4 @@
 #!/usr/bin/env python3
-# -*- coding:utf-8 -*-
 # Copyright 2019 The Android Open Source Project
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -16,8 +15,6 @@
 
 """Wrapper to run pytest with the right settings."""
 
-from __future__ import print_function
-
 import errno
 import os
 import shutil