security/py-gssapi: enforce that exactly one GSSAPI_* option is set

You must set exactly one OPTION from among GSSAPI_BASE, GSSAPI_HEIMDAL,
and GSSAPI_MIT. This patch, converting that OPTION group from
RADIO to SINGLE will make the error much clearer in this case.

PR: 273769
Reported by: Oclair <jason@aventia.pw>
MFH: 2023Q3