this change partially reverts #34139
using relative import helps with readability and ease the pain to write
down the full parent module name
in #34139, all relative imports were replaced with full path, because we
were using following code to verify if the code is python3 compatible:
```
mod_spec = importlib.util.spec_from_file_location(mod_name, path)
mod = importlib.util.module_from_spec(mod_spec)
mod_spec.loader.exec_module(mod)
```
but this does not work with submodule which can import using relative
import without specifying the name of the package and its parent module.
Signed-off-by: Kefu Chai <kchai@redhat.com>