Update network.pyi a bit

[elmot]

Fixes #241

[lancelote]

@elmot Where did you get country() and hostname() function signatures?

[elmot]

@elmot Where did you get country() and hostname() function signatures?

https://github.com/micropython/micropython/blob/master/docs/library/network.rst#network-functions

[elmot]

It does not return value if a parameter is provided

>>> import network
>>> print(network.country())
XX
>>> print(network.country('FI'))
None
>>> print(network.country())
FI
>>> 

[lancelote]

If nothing is returned, then the return type is None. NoReturn means the function always raises an exception or aborts the program execution.