Adopting the backport, I believe I've found a defect. I filed the defect with the backport as [backports.pdb 1](https://github.com/jaraco/backports.pdb/issues/1).

Essentially, the issue is that `__package__` gets set to the module name instead of the package name, causing issues with `from .mod` imports.

Feel free to take the relevant parts of the fix in the backport to adjust the implementation here, and definitely let me know if you think the adjustment I've made is somehow incorrect.

Full disclosure, I have not tested the canonical implementation in Python 3.7, only the backport in Python 3.6 and 2.7.

Please let me know if you would like me to file a separate issue.