DOC/TST: assorted flake8 and docstring improvements

[aqw]

This PR is eclectic, but it all relates.

• brings comments into the flake8 config
• escapes "``ASSET``\ s" consistently (needed for Python 3.12; flake8; unknown future Python version)
• converts all docstrings and help text into raw strings
• retires one ignored rule from flake8 (thanks to the above fixes)
• bumps RTD to Python 3.12 (thanks to the above fixes)
• makes sphinx render docstrings for __init__ and __call__
• improves the docstring for StoreKeyValuePairs
• improves the error message generated by StoreKeyValuePairs
• adds a test for conflicting number of keys passed to onyo new

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 95.93%. Comparing base (47baed3) to head (6b8f556).

Files	Patch %	Lines
onyo/argparse_helpers.py	83.33%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #612 +/- ## ========================================== + Coverage 95.76% 95.93% +0.17% ========================================== Files 67 67 Lines 5614 5614 Branches 1099 1097 -2 ========================================== + Hits 5376 5386 +10 + Misses 139 129 -10 Partials 99 99 ```

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