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).

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