perf python: Move and clean up parallel-perf.py

Move parallel-perf.py from tools/perf/scripts/python/ to
tools/perf/python/ as it is a standalone Python utility that invokes
'perf script' in parallel across time slices and CPUs. Update
tools/perf/tests/shell/script.sh accordingly.

Also fix bugs and clean up the script to pass mypy and pylint without
suppression comments:
- Fix Work.command() to return the shlex.quote()-escaped command string
  (previously sh_cmd was computed with shlex.quote() and discarded in
  favor of unquoted self.cmd).
- Close self.popen.stdout in the parent process after spawning the
  consumer subprocess in Work.start() when --pipe-to is used, ensuring
  the producer receives SIGPIPE if the consumer exits early and avoiding
  leaking the pipe file descriptor.
- Convert method and function names to snake_case, specify utf-8 file
  encodings, and add type annotations and docstrings.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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