[Stubs] Fix os.uname return value

[microbit-matt-hillsdon]

We've typed os.uname's return value as a string but it is a richer value with extra detail.

Error in program that runs correctly:

raised via https://support.microbit.org/a/tickets/53361

REPL:

>>> import os
>>> os.uname()
(sysname='microbit', nodename='microbit', release='2.0.0', version='micro:bit v2.0.0+16d4380 on 2021-10-25; MicroPython 16d4380 on 2021-10-25', machine='micro:bit with nRF52833')
>>> type(os.uname())
<class 'tuple'>
>>> os.uname().version
'micro:bit v2.0.0+16d4380 on 2021-10-25; MicroPython 16d4380 on 2021-10-25'
>>> len(os.uname())
5

Potentially we can use the typeshed version: https://github.com/python/typeshed/blob/master/stdlib/os/__init__.pyi#L450