Import numpy or skip for test_spatial_btree

test_spatial_btree currently fails if numpy is not available.

test_p_convergence_verifier uses the pytest importorskip function
to skip the test when numpy is not available.

Do the same for test_spatial_btree.

Fix prefork for Python 2.6.

Async Support For Prefork

This removes the deprecated function call_capture_stdout and
contains some whitespace tweaks.