Migrate project to a modern build backend

[benhoyt]

This would modernise things and also let us drop MANIFEST.in (cleaning up the root directory even further; see #2734).

[cristiangirlea]

I'd like to take this on. Since ops-tracing already uses Hatchling, would you be happy with migrating the root ops package to Hatchling too, preserving the current version source and wheel/sdist contents while removing MANIFEST.in? Should ops-scenario be included or handled separately? I'd verify builds from the sdist and that all three packages still install together.

[benhoyt]

Sorry, we'd like this to be done by @MattPatchava, a new joiner on the team. Assigned to him.