Currently the boolean debugger flag in the kernelspec file is determined at build time depending upon whether debugpy is installed or not. I think it should always be True, on the basis that ipykernel supports a debugger in principle and the details of the available debugging are determined at runtime.

This change has no effect on PyPI wheels or conda packages as the flag is always True in those, it only affects building from source. Because debugpy is a compulsory dependency in the pyproject.toml the flag is always True except when doing an editable install with build isolation (pip install -e .) in which case it is False. This is the recommended approach for installing from source specified in the README.md, and is how I came across this problem.

I have also updated test_debugger.py. There is a CI run that explicitly doesn't have debugpy installed and runs test_debugger.py, but unfortunately in this situation all the tests in that file are skipped. So this CI run only really tests that there is no debugger-related functionality in any of the other test files. I have changed test_debugger.py so that all tests are run regardless of whether debugpy is installed or not, and the asserts are different in each situation. Often without debugpy installed the tests don't do anything particularly interesting, but they do confirm that ipykernel doesn't block, or segfault, or raise an error in the non-debugpy situation.

The test_debugger.py tests are a little less readable now because of the new if statements. An alternative approach would be to keep test_debugger.py as it is and add a new test_not_debugger.py file.