When "python -m unittest discover" is run in a folder that is an implicit namespace package with the structure as below, no tests are discovered.
The condition that the tests must be importable from the top level directory is fulfilled and has been tested by importing the tests from the top level.

I did some investigating and have a PR underway that seems to fix it

Example project structure is:
.
├── requirements.txt
├── main.py
├── tests
    ├── unit
    │   └── test_thing1.py
    └── integration.py
        └── test_integration_thing1.py