Minor improvements and bug fixes:
- 🐛 FIX: retrieve future exception on_killed
The exception set on the future should be retrieved, otherwise it will be caught by the loop's exception handler.
- 🐛 FIX: Clean-up process event hooks:
On Process close/cleanup event hooks are removed,
in part to not persist cyclic dependencies of hooks <-> Process.
Once a process is closed, it will also not raise an Exception if a hook tries to un-register itself (but has already been removed by the clean-up).
- 👌 IMPROVE: Add `Process.is_killing` property
- 👌 IMPROVE: remove RUNNING from allowed states of `resume`:
Since there is no `resume` method implemented for the `Running` class.
- 🔧 MAINTAIN: Remove frozendict dependency