Make sure the hook files are executable.

In the repo's top directory on the development machine, do

```
$ git config --local core.hooksPath .githooks/
```

This sets the custom hooks directory. (The default is `.git/hooks`.)

This is usually done for you when you run `run` in the repo.

The hooks needs several Python packages. Make sure they are installed by `python3 -m pip install`.
