fix(internet): IGMPv3 and IGMP base id() disagree with the family-grouping ruling #2512

On main (94185da93), IGMP.id() is ('IGMP', 'IGMPv1', 'IGMPv2', 'IGMPv3') and IGMPv3.id() is ('IGMP', 'IGMPv3'). protocol-family-grouping.rst says the base is ('IGMP', 'IGMPv2') and that each member narrows to itself, per the ruling behind #2506. #2465 and #2506 both edited igmp.py, and the 4-tuple is what survived the two merges.

Fix, following the written ruling

• IGMPv3.id() becomes ('IGMPv3',), since RFC 3376 names it with a version.
• IGMP.id() becomes ('IGMP', 'IGMPv2', 'IGMPv3'). No member's id() is 'IGMPv1' any more; that selector still works through its registry key.
• The doc's member-shapes table and its Canonical Names section gain IGMPv3.

If the 4-tuple was deliberate at merge time, say so here and this closes as intended.