tools/pyboard.py: Support opening serial port in exclusive mode.
tools/pyboard.py: Support opening serial port in exclusive mode.

This is now the default, but can be overridden with CLI `--no-exclusive`,
or constructing `Pyboard(..., exclusive=False)`.

Signed-off-by: Damien George <99e6b749acbfbe2a596df99e91d24d6e1fdbee00@micropython.org>

henriknelson/micropython,adafruit/circuitpython,bvernoux/micropython,bvernoux/micropython,henriknelson/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,henriknelson/micropython,bvernoux/micropython,bvernoux/micropython,henriknelson/micropython,henriknelson/micropython,adafruit/circuitpython,bvernoux/micropython