ENH: Slim the mandatory dependency set: sympy and requests

Problem

Two of our five runtime dependencies serve one peripheral call site each, and both imports are already function-local:

• sympy (heavy; pulls mpmath, roughly 40 MB) is imported once, lazily, at quad.py:171, to run sym.primerange(0, 7920) — the first 1,000 primes for Weyl/Haber equidistributed sequences in qnwequi, and only when the caller didn't pass equidist_pp.
• requests serves only util/notebooks.py::fetch_nb_dependencies, the notebook support-file downloader.

Every pip install quantecon pays both costs; neither touches the numerical core. Because the imports are already lazy, moving them behind extras is close to mechanical.

Proposed change

• sympy: replace the call with a ~10-line sieve or a hardcoded constant array (the primes are fixed; regression-test the first/last values against the sympy output) and drop the dependency entirely. Fallback option: an extras group quantecon[quad] with a clear ImportError message.
• requests: move to an extras group (quantecon[notebooks]) with a lazy import and actionable error — or, if fetch_nb_dependencies has no remaining constituency, fold its retirement into the #786 utility-deprecation conversation (weigh the open fix issue for that function first).

Acceptance criteria

• Fresh-venv pip install quantecon no longer installs sympy (and, per decision, requests); import quantecon and the full suite green in that venv
• qnwequi output unchanged (regression test on default equidist_pp)
• Changelog notes the lighter footprint and any new extras

From the July 2026 technical-debt audit (AI-assisted; claims verified against 28d4b3b on 2026-07-25).

Start with the lazy import and prime generation at quad.py:171, then inspect util/notebooks.py::fetch_nb_dependencies and the open #786 utility-deprecation discussion before choosing the requests path. Verify the packaging metadata and run a fresh-venv install, the full suite, and qnwequi regression coverage; done means the unnecessary runtime dependencies are absent or isolated in documented extras and the changelog is updated.