[test] Remove OrderedDict usage

[sbc100]

Builtin dicts are guaranteed ordered as of 3.7 and we require 3.10 these days.

Also, document the 3.10 requirement in the README file and make the runtime
checks consistent.

[kripken]

Where do we require 3.10? I see that in a few specific scripts, but not in the main check.py or README. lgtm with docs in the README.

[sbc100]

Where do we require 3.10? I see that in a few specific scripts, but not in the main check.py or README. lgtm with docs in the README.

$ git grep sys.version
scripts/fuzz_opt.py:assert sys.version_info >= (3, 10), 'requires Python 3.10'
scripts/gen-s-parser.py:if sys.version_info < (3, 10):  # noqa: UP036

[sbc100]

The requirement for 3.10 looks like it was added in #8125

[kripken]

Makes sense, yeah, I just think we should document it outside of those 2 side scripts, in a prominent place, so it's clear it is a requirement of the entire project.

[sbc100]

It also in our .ruff.toml file: `target-version = "py310"

[sbc100]

Done