Dropping six.

[SylvainCorlay]

It seems that if py::exec get fixed (with respect to calling other functions defined in the same call), we should be able to

• drop the custom six-based exec function.
• drop the workaround due to the extra function in the traceback.

Also, six is used for selecting between PY3 and PY2 in is_complete_request.

[martinRenou]

pybind/pybind11#1654

[martinRenou]

The workaround when using py::exec would be to add a __builtins__ key to the globals argument containing the content of the builtins module if it's not already there. And in the case of xis_complete, we should use the same dict for globals and locals.

[martinRenou]

Fixed