At the moment you need to retrieve the key, then use its enable method:

key = gitlabProject.keys.get(12)
key.enable() 

No enable method exist on the manager object (keys in your example).

Sorry for getting back so late.
I tried your suggestion, but gitlab will give me a 404 error for for this:

key = gitlabProject.keys.get(12)

The request generated is:

/api/v3/projects/1/keys/12?project_id=1

The with that id is present, just not used in that project yet

I do think this is broken currently. There seems to be no way to enable a deploy key for a project. I'm also getting a 404 on .get().

I guess the get call for the key is not really supported for the gitlab api. The only way to do it seems to be:
https://docs.gitlab.com/ee/api/deploy_keys.html#enable-a-deploy-key
which would map to:
gitlabProject.keys.enable(12)
but that is not working due to the exception:
AttributeError: 'ProjectKeyManager' object has no attribute 'enable'