Historically, the kgb unit tests made use of nose as the test runner.

Unfortunately, nose 1.x is no longer maintained and is incompatible

with modern versions of Python 3. The forks are no better,  and nose2

is a very different library.
This change moves the codebase over to pytest. This is a much more

modern test runner with a lot of interesting capabilities and a wide

ecosystem of plugins.
Developers can now run pytest directly to run the test suite. The old

./tests/runtests.py still works, but is deprecated, and merely wraps

pytest anyway.

Unit tests pass on all supported versions of Python.