Using module 'subprocess' with timeout

Here's the Python code to run an arbitrary command returning its stdout data, or raise an exception on non-zero exit codes:

proc = subprocess.Popen(
    cmd,
    stderr=subprocess.STDOUT,  # Merge stdout and stderr
    stdout=subprocess.PIPE,
    shell=True)

communicate is used to wait for the process to exit:

stdoutdata, stderrdata = proc.communicate()

The subprocess module does not support timeout--ability to kill a process running for more than X number of seconds--therefore, communicate may take forever to run.

What is the simplest way to implement timeouts in a Python program meant to run on Windows and Linux?