Rollup merge of rust-lang#43046 - milmazz:bootstrap-unittest, r=aidanhs

bootstrap: Add doctests and unitests

This commit includes the following changes:

* Include more docstrings in classes, methods, and functions
* Add doctests, which are great for self-documenting our source code
* Add some unit tests with the `unittest` module
* Remove `WindowsError` reference on non-windows systems
* Rename some variables to be more explicit about their meaning
* Move all the attributes defined outside of `__init__`
* Add initial support for Python 3

r? @alexcrichton