Previously, to allow other threads to acquire the GIL, we would
just do a "releaseGIL(); acquireGIL();" and hope that another thread
would grab it.  The current thread has the best chance of grabbing the
GIL again, so I think what's been happening is that it would tend to
win the race to reacquire and starve all the other threads.

Now, the current thread is forced to wait for at least one other
thread to acquire the GIL before it can reaquire it.  This means that
we're at least fair between two threads, though not necessarily fair
between more than that (but at least it's more random and not as stacked
towards being unfair).