Auto merge of #45138 - johnthagen:future_imports, r=nikomatsakis

Add more __future__ imports to increase compatibility with Python 3 in bootstrap

The functionality of the  `__future__` imports are described [here](https://docs.python.org/3/library/__future__.html).

 These will help ensure the bootstrap code stays compatible with Python 3. If changes are made in the future that use absolute imports, division, or the `print` function, this will be ensure that running it under Python 2 will pass or fail the same way as Python 3.

`Option` is made a [new-style class](https://docs.python.org/2/reference/datamodel.html#new-style-and-classic-classes), so that it behaves the same way in Python 2 and 3.

The `__future__ unicode_literals` import is not used, because that can change the semantics of the code in Python 2 in unwanted ways. For more information see [this article](http://python-future.org/unicode_literals.html).

src/bootstrap/bootstrap.py

@@ -8,7 +8,7 @@
 # option. This file may not be copied, modified, or distributed
 # except according to those terms.
 
-from __future__ import print_function
+from __future__ import absolute_import, division, print_function
 import argparse
 import contextlib
 import datetime

src/bootstrap/bootstrap_test.py

@@ -10,6 +10,7 @@
 
 """Bootstrap tests"""
 
+from __future__ import absolute_import, division, print_function
 import os
 import doctest
 import unittest

src/bootstrap/configure.py

@@ -11,6 +11,7 @@
 
 # ignore-tidy-linelength
 
+from __future__ import absolute_import, division, print_function
 import sys
 import os
 rust_dir = os.path.dirname(os.path.abspath(__file__))
@@ -20,7 +21,7 @@ sys.path.append(os.path.join(rust_dir, "src", "bootstrap"))
 import bootstrap
 
 
-class Option:
+class Option(object):
     def __init__(self, name, rustbuild, desc, value):
         self.name = name
         self.rustbuild = rustbuild