Sync `Enterprise` class with API spec (#3342)

Turns `Enterprise` into a proper PyGithub object. Renames `environment`
arguments to `slug`.

This is a breaking change. If you used to call

    github.Github().get_enterprise(enterprise="…")

instead use one of the following calls now:

    github.Github().get_enterprise("…")
    github.Github().get_enterprise(slug="…")