Actually, I don't this is a bug: match_hostname() expects str data, and therefore IDNA-decoded domain names:

>>> b"xn--gtter-jua.example.de".decode("idna")
'götter.example.de'

Doing the matching on the decoded domain name should be safe.
Then it very much depends on whether the data you've got was IDNA-decoded, or naïvely ASCII-decoded, and I don't think the Python stdlib is very consistent here. Looking at the socket module, gethostbyaddr and getnameinfo seem to use ASCII decoding...