Timeout arg for launching tor broken on windows

Python's signal module lacks a 'SIGALRM' attribute on windows, causing the launch_tor function [1] to die with an AttributeError. We should look for an alternative for function timeouts that work on windows. Caught by pythonirc101.

[1] https://gitweb.torproject.org/stem.git/blob/HEAD:/stem/process.py#l16

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