We require Python >= 3.9 now, so there's no reason to avoid this any
more.

Signed-off-by: Colin Watson <cjwatson@debian.org>

Anything with a suitable `collect` method will do: for instance, it's
sometimes useful to be able to define a class whose `collect` method
yields all metrics from a registry whose names have a given prefix, and
such a class doesn't need to inherit from `CollectorRegistry`.

Signed-off-by: Colin Watson <cjwatson@debian.org>