Update minimum supported versions of frameworks in next major release

[sirosen]

In the next major webargs release (9.0), we should make sure to update the minimum versions of the various frameworks which we test against.

IIRC, the current bounds came from looking at which versions we support in practice. But we don't want to be stuck testing against old flask, django, pyramid, etc versions indefinitely. We could debate exactly what kinds of updates are safe in a minor release -- especially since this is for our testing configuration and not part of our install_requires -- but I think it's simpler to just do this in 9.0, whenever that may be.

[lafrech]

Some would say if the cost is low why not keep them, but the point is that old framework versions might not even be supported anyway so let's not try to support them here. Also the transition may be easy. I mean if someone maintains a 2018 app, I don't think they'll complain about webargs not supporting Flask 1, they'll just update Flask, the move is easy enough.