Keep skipping buffer tests on pypy

Adding numpy to the pypy test exposed a segfault caused by the buffer
tests in test_stl_binders.py: the first such test was explicitly skipped
on pypy, but the second (test_vector_buffer_numpy) which also seems to
cause an occasional segfault was just marked as requiring numpy.

Explicitly skip it on pypy as well (until a workaround, fix, or pypy fix
are found).