Previously we were limited to printing traceback starting down from just
storeblk() via explicit PyErr_PrintEx() - because pybuf was attached to
memory which could go away right after return from C function - so we
had to destroy that object for sure, not letting any traceback to hold a
reference to it.

This turned out to be too limiting and not showing full context where
errors happen.

So do the following trick: before returning, reattach pybuf to empty
region at NULL, and this way we don't need to worry about pybuf pointing
to memory which can go away -> thus instead of printing exception locally
- just return it the usual way it is done with C api in Python.

NOTE In contrast to PyMemoryViewObject, PyBufferObject definition is not
public, so to support Python2 - had to copy its definition to PY2 compat
header.

NOTE2 loadblk() is not touched - the loading is done from sighandler
context, which simulates as if it work in separate python thread, so it
is leaved as is for now.