BF: executable scripts should have shebang, the rest shouldn't be executable

Achieved by
```
find -perm /+x -type f | xargs grep -l '__main__' | xargs grep -L '#!' | xargs sed -i -e '1i#!/usr/bin/env python'
find -perm /+x -type f | xargs grep -L '#!' | xargs chmod a-x
```
with minimal review so ideally someone should have a look if no false positives