Debian used the new setters to disable TLS 1.0 and 1.1 in testing, bpo-31453. The old TLS versions have been enabled again for now. Python must expose the new API in case Debian decides to disable them again.
I also like to deprecate the old OP_NO_TLSv1 et al. flags in favor of the new API. The option flags are awkward to use and easy to get wrong. For example applications must not leave holes in the OP_NO range (e.g. allow TLS 1.0 and 1.2 but disable 1.1).