More dependency tending

[blast-hardcheese]

Why

As far as I can guess, our dependency pinning seems to have prevented dependabot from tending older libraries. Free these, as well as reshuffle dependency groups. Having Sphinx broken out to docs means that should we need to, we can explicitly install on an older Python runtime without having to worry about Sphinx needing Python>=3.9.

What changed

• Reflect recommended urllib3 range
• Switching from ^ to >= for dev dependencies

Test plan

• Ran unittest against 3.8

Rollout

• This is fully backward and forward compatible

[ryantm]

Thank you. Which dependencies are we hoping Dependabot will tend to? urllib3 or the dev ones?

[blast-hardcheese]

@ryantm Moving towards "all of them" from "few of them"

[blast-hardcheese]

Ideally, if CI passes we should be able to merge the PR. With sufficiently sensible dep ranges it should help people write better/more secure services.