New features
- Support icon-nickname-switch (248)
- Support LINE emoji in messages (255)
- Support Get user's language (256)
- Support get group summary, count and room count API (271, 272)
- Support emoji length and X-Line-Retry-key (270)
- Support python3.8
- Improve `handler.add` (259, 276)
See also [the milestone](https://github.com/line/line-bot-sdk-python/milestone/1?closed=1)