CircuitPython UART readline drops LF and returns None for blank lines #318

pslab.bus.busio.UART.readline() on main 08de0d0 consumes the LF byte before adding it to the returned data. A received blank line (b"\n") therefore returns None, just like a read timeout; b"hello\r\n" returns b"hello\r".

The method documents a line ending in a newline, matching CircuitPython UART.readline. pySerial also includes the LF terminator, allowing callers to distinguish a complete line from a partial timeout result.

I reproduced this through the real UART status/read command path with controlled firmware replies and a controlled clock, without physical hardware. Five cases fail on unchanged main: blank LF, ordinary LF, CRLF, binary data followed by LF, and successive-line framing. Five controls for timeout, partial lines, raw reads, readinto and zero-length reads pass.

I would like to fix this by appending the received byte before testing for the line terminator. The local fix passes all ten cases and 33 existing hardware-independent tests, configured source lint, wheel/sdist builds and Sphinx documentation. It leaves constructor/parity handling, polling and partial-timeout behavior unchanged; this is separate from #317.

AI assistance: Codex helped investigate the contract, prepare the minimal fix and run command-level tests. No physical UART integration test is claimed.