Python 3.6.x support is now dropped (v.1.0.0)

[simon-ritchie]

The apysc has dropped Python 3.6.x support since it reached EOL. So from now on, the apysc supports Python 3.7, 3.8, 3.9, and 3.10.

And this change became a destructive one, incremented the major version to 1.0.0.