PyObject_SetAttr() and PyObject_GetAttr() now also accept Unicode

objects for the attribute name. Unicode objects are converted to
a string using the default encoding before trying the lookup.

Note that previously it was allowed to pass arbitrary objects as
attribute name in case the tp_getattro/setattro slots were defined.
This patch fixes this by applying an explicit string check first:
all uses of these slots expect string objects and do not check
for the type resulting in a core dump. The tp_getattro/setattro
are still useful as optimization for lookups using interned
string objects though.

This patch fixes bug #113829.