This diff remove shebangs from python modules that are supposed to be imported rather than executed on their own.

For now this doesn't touch the individual functional tests, that are technically standalone but are always executed via test_runner.py with the python interpreter explicitely used (python <path>.py). This can be done in a separate diff, as it also involves changing the file permissions.

Depends on D15432

This will be covered by a new linter that I'm working on locally. Just check that the change makes sense and that all the files touched here don't have executable permission.