Align RuleList with Python 3.14 and modern dependency/runtime expectations

[Copilot]

Python 3.14 support and packaging needed minimal, targeted updates to keep the library usable and buildable.

• Python target & packaging: Raised classifiers/python_requires to 3.14 and refreshed requirements (numpy/scipy/sklearn/pandas/numba) without adding new libraries beyond numba.
• Pandas 2.x compatibility: Replaced deprecated iteritems usage and preserved nominal category ordering.
• Optional acceleration: Added a safe numba import fallback so environments without numba still run.
• Legacy API: Restored _fit_rulelist helper with documented parameters for backward compatibility.
• Beam search behavior: Parameterized/documented deterministic shortcut and depth override for legacy expectations.

Example usage of deterministic beam controls:

from rulelist.search.beam.itemset_beamsearch import find_best_rule

rulelist.use_deterministic_shortcut = False  # enable full beam search
rulelist.max_search_depth = rulelist.max_depth
subgroup = find_best_rule(rulelist, data)

Original prompt

Make a plan to update the whole repository with the latest python version without introducing new packages.

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