[jonathan-taylor]

ModelSpec has moved to its own package, modelmatrix (0.3.0 on PyPI). ISLP now depends on modelmatrix>=0.3 instead of carrying its own copy.

Imports keep working.

• from ISLP.models import ModelSpec (and poly, ns, bs, pca, derived_feature, contrast, ordinal, ...) gives modelmatrix's objects.
• ISLP.models also exports modelmatrix's wildcard (R's .), Contrast, Ordinal and its responses Surv, Binomial, Multinomial and Outcome.
• ISLP.models.model_spec, ISLP.models.columns and ISLP.transforms are aliased to modelmatrix's modules in sys.modules, so from ISLP.transforms import Poly and similar still work, private names included.

Breaking, as 0.6.0 announced: intercept defaults to False. statsmodels' models, and sklearn_sm / sklearn_selected with them, need intercept=True. The 0.6.0 FutureWarning about the move is gone.

Bug fix in strategy.py. Stepwise selection counted terms with model_spec.terms and mapped them to terms_ by position. With a wildcard, which is one term standing for many, the two lists don't line up, and selection failed. It now counts terms_ and maps given terms (lower_terms, upper_terms, initial_terms) to their fitted versions through features_. A column the wildcard stands for can therefore be a lower term. The unused imports of the column helpers, and the unused _build_submodel, are removed.

Tests:

• The ModelSpec tests moved to modelmatrix.
• tests/models/test_modelmatrix.py checks the re-exports, the old import paths and the new default. It also checks stepwise selection with a wildcard: forward, a lower term among the wildcard's columns, and fixed-step backward. Those three tests fail on the old strategy.py.
• The two stepwise tests that were in the old ModelSpec tests moved here: a strict xfail for derived terms, and the 1-SD rule.
• Locally: 58 passed, 2 xfailed (without the network test).

Docs:

• The ModelSpec and transforms pages now live in modelmatrix's docs. ISLP's sidebar keeps the entries, as links there: spec, ANOVA, comparison, responses, poly, splines and PCA. models/selection stays an ISLP page. The moved notebooks are removed.
• intersphinx maps modelmatrix's API, so a reference like :py:class:modelmatrix.ModelSpec`` links to its API page.
• The API list drops ISLP.models.model_spec, ISLP.models.columns and ISLP.transforms, which are now modelmatrix's modules.
• Checked with a local Sphinx build (notebooks not executed), using the objects.inv of a local modelmatrix docs build. The reference resolves, no page links to a removed one, and there are fewer warnings than before.
• The links and the inventory need modelmatrix's site to be deployed (one manual run of its Build docs workflow). Until then, an ISLP docs build gets an intersphinx warning, and the links return 404.

Not in this PR:

• The labs (ISLP_labs) need intercept=True wherever they rely on the old default, plus the Ch07 ordinal('education') fix. That's a separate PR there.

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