Clean up the build files

[emmamarichal]

This is my first PR here, we a simple task (in appearance...).

This should solve this issue: #875

I have no idea of the working process here, so please feel free to correct me and tell me how it works. I'm curious :)

@simoncozens @m4rc1e

[simoncozens]

Looks good, can you run black Lib/gftools/builder/__init__.py and commit the results - that will make the tests happier.

[simoncozens]

Thank you!