RFC: cleanups & misc changes

Hello,

following this is a bunch of 10 fairly trivial cleanups & pylint warning
fixes for {btt,bno}_plot.py

Follow-Ups:
- Re: RFC: cleanups & misc changes
  - From: Jens Axboe
- [PATCH 10/10] btt_plot.py: Use `with open() as ...` context manager
  - From: Vincent Legoll
- [PATCH 09/10] bno_plot.py: Fix pylint: singleton-comparison
  - From: Vincent Legoll
- [PATCH 08/10] bno_plot.py: Fix pylint: len-as-condition
  - From: Vincent Legoll
- [PATCH 07/10] btt_plot.py: Fix pylint: no-else-return
  - From: Vincent Legoll
- [PATCH 06/10] btt_plot.py: Fix pylint: singleton-comparison
  - From: Vincent Legoll
- [PATCH 05/10] btt_plot.py: Fix pylint: len-as-condition
  - From: Vincent Legoll
- [PATCH 04/10] btt_plot.py: Fix pylint: wrong-import-order
  - From: Vincent Legoll
- [PATCH 03/10] bno_plot.py: Use `with open() as ...` context manager to automatically handle close()
  - From: Vincent Legoll
- [PATCH 02/10] bno_plot.py: Use shutil.rmtree() instead of os.system('/bin/rm')
  - From: Vincent Legoll
- [PATCH 01/10] btt_plot.py: Use sum() instead of open-coding it to compute list average
  - From: Vincent Legoll