fix: fetch() selects message body by structure, not length (v0.1.3)

select the literal payload by isinstance bytearray instead of len>20. aioimaplib
stores the message body as the only bytearray in the response; every other line
(including the '<id> FETCH (...' header) is plain bytes. the length heuristic
matched the header line first for any 2+ digit message id or BODY[]/UID fetch,
returning a blank Message and silently breaking OTP retrieval on real mailboxes.

Signed-off-by: disqualifier <dev@disqualifier.me>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "aiomail"
-version = "0.1.2"
+version = "0.1.3"
 description = "async IMAP one-time-code retrieval with password/OAuth2 auth and dynamic matching"
 requires-python = ">=3.10"
 dependencies = [