CI has started failing for Python 3.6: https://github.com/simonw/sqlite-utils/actions/runs/3576322798

It's fixable by swiching away from ubuntu-latest according to:

• Python 3.6 fails to setup actions/setup-python#355 (comment)

But https://endoflife.date/python says that 3.6 end of life was almost 6 years ago, and end of security support nearly 1 year ago.

So I'm OK dropping support entirely - Python 3.6 users will still be able to install version 3.30, just not any releases that come next.