[Part 1 of 2] [qa] receivedby.py cleanup and fixes
[Part 1 of 2] [qa] receivedby.py cleanup and fixes

Summary:
[qa] Tidy up receivedby.py
- Fix flake8 warnings
- Remove the useless get_sub_array_from_array() function
- Reduce runtime for receivedby.py by about half by only using two nodes

Backport of Core PR11055

Test Plan:
  make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1983

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