Drop support for Python 3.7

[simonw]

I just attempted to upgrade to the latest Black and it failed because Black no longer supports Python 3.7:

ff57a97

I'm going to drop 3.7. It hit EOL over six months ago - 2023-06-27 https://devguide.python.org/versions/

[simonw]

Of slight concern:

That's from https://pypistats.org/packages/sqlite-utils - it shows the proportion of downloads for each Python version.

How is Python 3.6 still 51% of my downloads!? That hit EOL in December 2021.

I guess those users are getting version 3.30, the last to support Python 3.6: https://sqlite-utils.datasette.io/en/stable/changelog.html#v3-30