fix: relax docstring-parser constraint to support Python 3.14

[umutonuryasar]

aisuite fails on import on Python 3.14 with:
AttributeError: module 'ast' has no attribute 'NameConstant'

This is caused by the docstring-parser dependency. Version 0.15 relies on ast.NameConstant, ast.Str, and ast.Num, which were deprecated since Python 3.8 and removed entirely in Python 3.14.

pyproject.toml pinned docstring-parser with a caret constraint:

docstring-parser = { version = "^0.15.0" }

Under Poetry's semantics for 0.x versions, ^0.15.0 resolves to >=0.15.0,<0.16.0, so the resolver could never pick up a release with the fix (the deprecated ast classes were removed in a later 0.x release; current latest is 0.18.0).

Relax the constraint to >=0.16,<1.0, allowing resolution to 0.18.0, which removes the deprecated ast usage. poetry.lock regenerated accordingly (docstring-parser 0.15 → 0.18.0).

Verified:

• import aisuite as ai succeeds with no AttributeError or DeprecationWarning.
• Full test suite: 348 passed, 1 skipped (pre-existing failures from missing optional extras such as mcp/cohere/deepgram are unaffected and identical with/without this change).

Fixes #265