Return more union classe like `NamedUser | Organization | Enterprise` (#3224)

More attributes return a `Union` of PyGithub classes:
- `CommitComment.user` returns `NamedUser | Organization`
- `CommitStatus.creator` returns `NamedUser | Organization`
- `DependabotAlert.dismissed_by` returns `NamedUser | Organization |
None`
- `Deployment.creator` returns `NamedUser | Organization`
- `GithubApp.owner` returns `NamedUser | Organization | Enterprise`
- `Invitation.invitee` and `Invitation.inviter` return `NamedUser |
Organization`
- `IssueComment.user` returns `NamedUser | Organization`
- `IssueEvent.actor`, `IssueEvent.assignee`, `IssueEvent.assigner`,
`IssueEvent.requested_reviewer` and `IssueEvent.review_requester` return
`NamedUser | Organization`
- `Migration.owner` returns `NamedUser | Organization`
- `Project.creator` returns `NamedUser | Organization`
- `StatsContributor.author` returns `NamedUser | Organization`