Adding markers to group Python benchmarks

We should be able to group the Python benchmarks by, for example, schedulers. The pybench marker can be used to filter benchmarks, and we already have one marker, inner_outer_persistent, which allows us to run all the benchmarks with the marker by pytest benchmarks/python/ -m inner_outer_persistent. This maker seems like a reasonable way to categorize the benchmarks. Some benchmarks do use multiple schedulers, for which we could probably just add multiple markers.