Issue 46508: codec name acceptance became way too lenient in 3.9

in 3.8 this was not a valid codec name: "เ_เ_เ_iDnA"
in 3.9 it gets treated as idna and triggers the punycode decoder when passed to bytes.decode(codec).

Discovered by oss-fuzz.

_Likely_ a consequence of https://bugs.python.org/issue37751

The consequences of this change are that anyone can stuff heinous strings into codec names and get a non-LookupError behavior out of them. Anywhere codecs can be part of user input this has many interesting potential negative consequences.

<=3.8 gave `LookupError("unknown encoding: ...`