bpo-30835: email: Fix AttributeError when parsing invalid CTE (GH-13598)

bpo-30835: email: Fix AttributeError when parsing invalid CTE (GH-13598)

* bpo-30835: email: Fix AttributeError when parsing invalid Content-Transfer-Encoding

Parsing an email containing a multipart Content-Type, along with a
Content-Transfer-Encoding containing an invalid (non-ASCII-decodable) byte
will fail. email.feedparser.FeedParser._parsegen() gets the header and
attempts to convert it to lowercase before comparing it with the accepted
encodings, but as the header contains an invalid byte, it's returned as a
Header object rather than a str.

Cast the Content-Transfer-Encoding header to a str to avoid this.

Found using the AFL fuzzer.

Reported-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Andrew Donnellan <andrew@donnellan.id.au>
* Add email and NEWS entry for the bugfix.
(cherry picked from commit aa79707262f893428665ef45b5e879129abca4aa)

Co-authored-by: Abhilash Raj <maxking@users.noreply.github.com>