ENH: Avoiding NPY_BEGIN_THREADS for small arrays can speed-up trivial_three_operand_loop by 5%

In <code>trivial_three_operand_loop</code> from <b>ufunc_object.c</b>, avoiding NPY_BEGIN_THREADS for small arrays, can speed-up function by <b>5%</b>.

As releases of GIL, then quickly restoring just after small operation doesn't provide much benefit.

Before
----------
<pre>
In [2]: x = np.asarray(5.0)
In [3]: timeit x+x
1000000 loops, best of 3: 1.99 us per loop
</pre>

After
-------
<pre>
In [3]: timeit x+x
1000000 loops, best of 3: 1.71 us per loop
</pre>