perf-python: import an event-counting script for perf

The script count_between.py retrieves tracepoints events saved into a
recorded perf archive, it sorts them according to their type and
counts events which occurred between two edges events.

For example, the following command counts the events sched_switch
which occurred between the tracepoints sys_enter and sys_exit:
perf script -s between.py \
    events=raw_syscalls:sys_enter,sched:sched_switch,raw_syscalls:sys_exit
perf-python: minor change in latency bin script