Style checker tries to check the style of README.txt.py in the build folder

For some reason, running `./dev style` throws a warning that it cannot parse the file README.txt.py in the build folder:
 `WARNING: Error in file ./build/img/at_home/unlocking-the-secret-in-product-codes/README.txt.py: Cannot parse file.`

To reproduce, run `./dev update` and then `./dev style`. The style check should probably be limited so as to not try and check for build folder.