FakePayload is a wrapper around io.BytesIO and is expected to
masquerade as though it is a file-like object. For that reason it makes
sense that it should inherit the correct signatures from io.BytesIO
methods.
Crucially an implementation of .readline() is added which will be
necessary for this to behave more like the expected file-like objects as
LimitedStream will be changed to defer to the wrapped stream object
rather than rolling its own implementation for improved performance.
It should be safe to adjust these signatures because FakePayload is
only used internally within test client helpers, is undocumented, and
thus private.
