`Installation.get_repos` needs to differentiate between IAT access and UAT access

get_repos calls /installation/repositories. This is the correct endpoint when the Installation was created from an IAT (or an app, which pygithub handles and upgrades internally).

However if the Installation was obtained via a user access token e.g. AuthenticatedUser.get_installations, then this triggers an HTTP 403 error, the correct endpoint is /user/installations/{installation_id}/repositories.

Sadly one can not even use the repositories_url which is returned as part of the installation object, because it's apparently hardcoded to /installation/repositories (note that it is not currently used by the code anyway, get_repos hard-codes that URL separately).

I can see two places to handle this, both by checking if the auth is AppUserAuth:

• lazily in get_repos, and switch to the user version
• or eagerly in __init__, after checking for AppAuth check for AppUserAuth, and update the _repositories_url (if necessary, in case github eventually fixes this?), and update get_repos to access self.repostories_url