RFC 4489.
Returned value is in printable notation.

'addr' parameter specifies the link-local address to use for generating
Link-scoped multicast address IID.

By default, the function returns a ::/96 prefix (aka last 32 bits of
returned address are null). If a group id is provided through 'grpid'
parameter, last 32 bits of the address are set to that value (accepted
formats : b'