To reduce complexity, I don't support old Bangumi URLs directly via
_VALID_URL. Instead, I choose to let it go to generic redirection. An
example can be found in #10190:
http://bangumi.bilibili.com/anime/v/40062
HTMLParser, which is used by extract_attributes, already unescapes
attribute values with HTMLParser.unescape. They shouldn't be unescaped
again, to there may be parsing errors.
Ref: #11219, #11522