util: fix the signature of observedbufferedinputpipe._fillbuffer()
Flagged by PyCharm, since it didn't match the signature of the method being
overridden. The default value in the superclass is also `_chunksize`, and I
suspect that the amount read from `osread` should be limited to what is passed
in. Only one caller (`bufferedinputpipe.unbufferedread()`) passes this
argument, and it passes the max of `_chunksize` and whatever it was passed.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true