Use in_notebook() function instead of IN_NOTEBOOK constant

[ncoop57]

Summary

Replaces the module-level IN_NOTEBOOK constant with calls to the in_notebook() function so the notebook check is evaluated at call time rather than at import time.

Changes

• nbdev/serve.py: proc_nbs now calls in_notebook() when choosing the parallel start method
• nbdev/test.py: test_nb and nbdev_test now call in_notebook() for SIGINT handling, IN_TEST env var, and start method selection
• nbdev/__init__.py: added a trigger note to the nbdev.skill module docs

Why

Evaluating at import time means the value is fixed regardless of the actual runtime environment (e.g. in spawned/forked workers, or when imported before a notebook kernel is established). Calling in_notebook() gives the correct answer at the point of use.