How to access partition key from HookContext?

[rprechelt]

How can I access the partition key of a partitioned asset when using the (newly added) hooks support?

e.g., given a HookContext in a hook attached to an asset, how do I access the partition_key?

@dg.success_hook(required_resource_keys={"slack"})
def slack_on_success(context: dg.HookContext):
    # how do I get access to the partition key here? 

@dg.asset(
    ...
    partitions_def=partitions,
    hooks={slack_on_success},
)