ModifyList when modifying config in package.py

Hello!

I'm trying to add a release_hook to my package to copy some scripts to set locations outside the Rez package structure (for Deadline).

Following what I've found on the wiki, I have this in my package.py:
```
with scope("config") as c:
c.release_hooks = ModifyList(append=["deadline"])
```

But every time I try to do a rez-build --install, I get this error:

> 11:40:35 ERROR ResourceError: Problem loading c:\users\brian.silva\packages\...\package.py: invalid syntax (package.py, line 52)

And if I then look at that file, I see this:
```
with scope('config') as config:
config.plugin_path =
```

Am I doing something incorrectly here?

Start by reproducing the reported `rez-build --install` command with the `package.py` snippet using `scope("config")` and `ModifyList`. Inspect the generated `package.py` shown in the report; done means the ModifyList value is serialized as valid package configuration and installation completes without the syntax error.

Written by the indexing model from the issue text.