Refactor PyNEST example: eprop_supervised_regression_infinite-loop.py

[gtrensch]

Pylint Static Code Analysis reports the following code quality issue:
eprop_supervised_regression_infinite-loop.py:640:0: R0917: Too many positional arguments (6/5)

This was causing builds to fail. Currently, we have lowered the fail-threshold for pylint (#3339) to allow merges to continue. The PyNEST example needs to be refactored to achieve the desired code quality.