python: refactor BSPY_REGISTER_ENUM to fix compatibility with Python 3
python: refactor BSPY_REGISTER_ENUM to fix compatibility with Python 3

Earlier `pybind11::module::import()` was used to obtain kernel's
`pybind11::module` object and that lead to reinitializing kernel againn
and again (with call to `load_plugins()`).
Fix this by using `kernel::self_pymod()` to directly access Python module
object.

uentity/blue-sky-re,uentity/blue-sky-re,uentity/blue-sky-re