RX and CTS are the input pins and pull-ups are enabled so they don't cause
a problem if left unconnected. But the output pins don't need a pull up
(they were originally all configured with pull up in commit
8f7491a109a555ca897ae49efe98f4cc2b080311).
If needed, the pull-ups can be disabled in Python using machine.Pin after
the UART is constructed.
See issue #4369.
Signed-off-by: Damien George <damien@micropython.org>