update-checkout: Explicitly use python3

This changes the shebangs for the update_checkout executable files to
specifically call Python 3. The code is already compatible and
functional with python3, so this removes any implied backward
compatibility, and removes any ambiguity based on the user's current
environment, especially since some systems, like macOS, still link
'python' to Python 2.

This also removes the now unnecessary 'from __future__' imports.

Author: jorng

utils/update-checkout

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import sys
 

utils/update_checkout/run_tests.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # This source file is part of the Swift.org open source project
 #
@@ -13,9 +13,6 @@
 Small script used to easily run the update_checkout module unit tests.
 """
 
-
-from __future__ import absolute_import, unicode_literals
-
 import os
 import shutil
 import sys

utils/update_checkout/update_checkout/update_checkout.py

@@ -8,8 +8,6 @@
 # See https://swift.org/LICENSE.txt for license information
 # See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-from __future__ import print_function
-
 import argparse
 import json
 import os