Rework dependencies, dev testing, run script and CI

[dpranke]

This reworks a bunch of logic in //.github/workflows/ci.yml and //run as follows:

I Dropped support for doing development with Python 3.8, 3.9, or 3.10;
the code still runs on those versions and will be tested on those versions,
but you have to have a newer version to build the package and run the other tools.
This restructures the dev dependencies in such a way that hopefully Dependabot
will stop being confused by issues in old versions of the dependencies.

I reworked the CI configuration on GitHub accordingly so that we don't try
to build the package or run the tools on old versions; as a result, there are two
sets of jobs: a set that builds the package using the latest Python (3.14 at the
time of writing) and then tests it using 3.8, 3.9, and 3.10, and a set that runs
presubmit to run all of the other checks on the other supported versions of
python.

I heavily rewrote //run to be more consistent and to provide cleaner logging
of what it is doing.

I also rewrote //README.md to hopefully be clearer and more helpful to anyone
else that might be trying to use or modify this package.

[dpranke]

Abandoning as it was merged in separately.