Threading support is still very new so stay conservative at this point
and enable threading without the GIL.  This requires users to protect
concurrent access of mutatable Python objects (eg lists) with locks at
the Python level (something you should probably do anyway).  The
advantage is that there is less of a performance hit for non-threaded
code, because the VM does not need to constantly release/acquire the GIL.

In the future the GIL will be made more efficient.  There is also room to
improve the efficiency of non-GIL code by not using mutex's if there is
only one thread active.