fix assertionerror with infinite loop in context manager

[jakkdl]

fixes #295

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flake8-async/flake8_async/visitors/visitor91x.py

Lines 43 to 46 in db5317c

	# Statement injected at the start of loops to track missed checkpoints.
	ARTIFICIAL_STATEMENT = ArtificialStatement("artificial", -1)
	# There's no particular reason why loops use a globally instanced statement, but
	# `with` does not - mostly just an artifact of them being implemented at different times.