chore(dev): let more tools know that we use 120 character per line (#10339)

Some instances of flake8 expect to find their `max-line-length` config
in `setup.cfg`.

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Author: apiarian-datadog

Diff for: setup.cfg

@@ -19,3 +19,6 @@ addopts =
 # DEV: The default is `test_*\.py` which will miss `test.py` files
 python_files = test*\.py
 asyncio_mode = auto
+
+[flake8]
+max-line-length = 120