tools/pyboard: Run exec: command as a string.

The Python documentation recommends to pass the command as a string when
using Popen(..., shell=True).  This is because "sh -c <string>" is used to
execute the command and additional arguments after the command string are
passed to the shell itself (not the executing command).

https://docs.python.org/3.5/library/subprocess.html#subprocess.Popen