Auto merge of #43003 - milmazz:bootstrap-pep8, r=alexcrichton

bootstrap: Fix all the pep-8 issues reported by flake8

This commit also adds a few missing docstrings.

Today, after reading this [article](https://blog.rust-lang.org/2017/06/27/Increasing-Rusts-Reach.html), I downloaded this project and started building from source. In the meantime, I began to read the `bootstrap.py`, to know more about the building process, and I made a few changes, this is my first contribution to the project, hope you like it.

BTW, I have a few doubts about the `bootstrap.py`, any guidance is more than welcome:

* Where can I find the unit tests for this script? In case it doesn't exist yet, do you like to include some unit tests with pytest?
* Some methods like `fix_executable`, `get_string`, and `exe_suffix` in the `RustBuild` class should be converted to a function because it doesn't use `self` anywhere. What do you think?

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