Fontbakery: Use Python PEP8 style 4-space indentation

[josh-hadley]

Change indentation to PEP8 style 4 spaces for consistency across other Python-based font projects (fontTools, AFDKO).

[felipesanches]

I am using the autopep8 module to make the bulk of the work. It is not perfect, but it saves me a lot of time. I'm then reviewing the result and fine-tunning.

[felipesanches]

this is the command I'm using:
autopep8 path/to/file.py --select=E101,E121 --in-place

[felipesanches]

I've also used the --recursive flag to apply the changes to the entire codebase.

[anthrotype]

I recommend using black autoformatter. It's become the de facto standard, used by many big projects including new modules in the Python standard library.

[felipesanches]

I considered using black, but ended up using autopep8 and now I am halfway done. I doubt the tools differ in such a specific purpose (I'm focusing exclusively on 4-spaces indentation in this PR) so I'll finish it with the same tool.

[felipesanches]

done! :-D