Fix misused pyproject.toml

1 job for develop in 45 seconds (queued for 1 second)
Name Stage Failure
failed
test Test
----------------------------- Captured stderr call -----------------------------
INFO:traitlets:Executing notebook with kernel: python3
INFO:traitlets:Executing notebook with kernel: python3
INFO:bookbook.latex:Combined 2 files
INFO:bookbook.latex:Converting to pdf
=========================== short test summary info ============================
FAILED tests/test_nbcourse.py::test_nbcourse_build - assert 2 == 0
======================== 1 failed, 20 passed in 26.34s =========================
ERROR: Job failed: exit code 1