Stop supporting py27, py34, py35

[okue]

https://pypistats.org/packages/line-bot-sdk indicates very few users use this sdk with py27, py34, py35.

If there is no problem, we want to stop supporting py27, py34, py35.
We can start to use new features such as asyncio, type hints :)

(Asycnio is discussed at #101)

[o926428377]

Although there were few users download the sdk py27, py34, py35, I though lots of users are still using some legacy sdk without update.
Suggest to monitor Line API Server if it could detect what the python version from request source.
btw. I'm the user of py27, py34 :)

[wasdee]

I think the development could be start right away.
Old users could just pin the package version.
since python lib version and line api version is independent of each other.

I use fastapi on my projects, asyncio, type hints are going to be benefit for me and all majorities as the stats suggest.

[tokuhirom]

https://endoflife.date/python
It was deprecated about 10 months ago. It's time to deprecate it.

[tokuhirom]

It was already removed :) 02a23fb