MAINT: remove the deprecated notebooks utility and its requests dependency (v1.0)

[mmcky]

Problem

requests and urllib3>=2 are still listed in environment.yml, and the notebooks optional-dependency group is still in pyproject.toml. Both exist only to keep the deprecated quantecon/util/notebooks.py working and testable through its final release.

They cannot be removed on their own: the CI test environment is built from environment.yml, and quantecon/util/tests/test_notebooks.py exercises fetch_nb_dependencies over the network while the module still ships. So this is gated on the module removal itself, and the whole set should land together in v1.0.

Background

fetch_nb_dependencies was deprecated in 0.12.0 (#953) after the July 2026 audit established it has no remaining constituency — no usage in any of the lecture repositories, and the callers that exist are dormant notebooks descending from two 2017-era demos. Its default remote, QuantEcon/QuantEcon.notebooks, was last pushed 2023-07-15. Data still required lives in QuantEcon/data-lectures.

Three PRs and one issue were closed on the strength of that decision: #870, #904, #905.

Tasks

• Delete quantecon/util/notebooks.py and quantecon/util/tests/test_notebooks.py
• Drop fetch_nb_dependencies from quantecon/util/__init__.py and from the top-level quantecon/__init__.py export list
• Delete docs/source/util/notebooks.rst and its entry in docs/source/util.rst
• Remove the notebooks optional-dependency group from pyproject.toml
• Remove requests and urllib3>=2 from environment.yml
• Check environment_np2.yml and the conda-forge feedstock for the same entries
• Confirm a fresh install and the full suite are green with neither package present

Acceptance criteria

• grep -rn "requests\|fetch_nb_dependencies" quantecon environment*.yml pyproject.toml returns nothing
• Full suite green in an environment with neither requests nor urllib3 installed
• Release notes record the removal and the lighter dependency set

Follows #953. Completes #880.