P3 got rid of all __private attributes in the `threading` module,
via python/cpython@d06489945f.
Our old code for forcing the `daemon` attribute on an already started
thread used the mangled private name and does not work anymore on P3.
We need to use the new private attribute name (actually both,
to keep backwards-compatibility w/ P2)
This might have deserved a pycompat counterpart, but setting both
variants of the attribute works with no hassle. It should not be a very
frequent use case either.
Olivier Dony
d03b4f8675