pyiso8601 1.0.0

I've released pyiso8601 1.0.0, my Python ISO 8601 date parsing library.

This is a big milestone, with lots of changes to the packaging and release. The interface remains unchanged.

The biggest change is I've dropped Python < 3.6 support. Now this is Python 3.6+ only.

With the move to Python 3+ I've added type annotations to the code instead of the .pyi files. I've also dropped some compatibility code, simplifying the internals.

I've also switched to poetry and nox for packaging and building. This makes no difference to anyone consuming this library but makes maintaining a little easier.