[FEAT] Adding Python 3.15

[henrydingliu]

Summary of Changes

adding python 3.15 to supported version
adding python 3.15 dependencies
addressing a numpy 2.5 deprecation warning

Related GitHub Issue(s)

#1478

AI/LLM Usage

used AI to locate the numpy deprecation warning

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---

Note

Medium Risk
Core triangle valuation date computation changed for NumPy compatibility, and 3.15 pulls in major dependency upgrades (pandas 3, newer numpy) that could affect behavior across installs.

Overview
Adds Python 3.15 to CI (pytest workflow matrix) and project classifiers, and splits dependency pins in pyproject.toml so 3.15+ gets newer stacks (e.g. pandas ≥3.0.6, numpy ≥2.5.2, scikit-learn ≥1.9.1, matplotlib/contourpy/numba, plus bumped optional deps like lxml, pyarrow, scipy, rpds-py).

Updates valuation date math in chainladder/core/base.py to use explicit timedelta64[M] offsets instead of adding integers to datetime64[M], addressing a NumPy 2.5 deprecation while keeping the same valuation index behavior.

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[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.6% of exported symbols fully typed (227 / 1456)

	Known	Ambiguous	Unknown	Total
Project (head)	227	108	1121	1456

Other symbols referenced but not exported by chainladder: 13

	Known	Ambiguous	Unknown	Total
Other (head)	3	1	9	13

Symbols without documentation:

• Functions without docstring: 326
• Functions without default param: 0
• Classes without docstring: 8

Patch (exported symbols added or changed by this PR): no exported symbol type-completeness changes detected.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.19%. Comparing base (856f87b) to head (1f6823c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1479      +/-   ##
==========================================
+ Coverage   94.15%   94.19%   +0.03%     
==========================================
  Files          96       96              
  Lines        5852     5852              
  Branches      739      739              
==========================================
+ Hits         5510     5512       +2     
+ Misses        221      220       -1     
+ Partials      121      120       -1     

Flag	Coverage Δ
unittests	94.19% <ø> (+0.03%)	⬆️

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