If the USB serial device locks up, then writes to that device can hang
forever. That can make the test runner `tests/run-tests.py` lock up, among
other problems.
This commit introduces a 5 second write-timeout, and catches any OSError's
raised during `enter_raw_repl()`. Now, if a USB serial device locks up,
`enter_raw_repl()` will eventually raise an exception.
Signed-off-by: Damien George <damien@micropython.org>
dea3035b88