Allow running RTM over a thread
Allow running RTM over a thread

When trying to start a RTM connection over a thread a RuntimeError appears:
RuntimeError: set_wakeup_fd only works in main thread
This change allows it.
slackapi/python-slackclient,slackhq/python-slackclient,slackapi/python-slackclient,slackapi/python-slackclient