This prepares pyselect codebase for future logic where all channel
element types will be sent via inplace _selcase data. For PyObject we
could previously go with "wiring ptx through pycase[1]", but for
arbitrary type, that has to be first converted from Python object to
C-level object, we would have to store the result somewhere, and that
would mean extra allocation and pyselect code complexity increase, even
for cases that don't use anything but chan[object].

So do the preparation and switch pyselect into using inplace tx.