[issue16353] add function to os module for getting path to default shell

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Taking into consideration msg174586 we can ignore the ComSpec environment 
variable and hardcode "cmd.exe" on Windows, as we ignore the SHELL environment 
variable on Posix systems.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue16353>
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