Drop Python 3.9 support and update type hints

[LeMyst]

Removed Python 3.9 support from workflows, documentation, and pyproject.toml. Updated type hints throughout the codebase to use PEP 604 union syntax (e.g., str | None) and cleaned up related imports. Minor formatting improvements and dependency updates for Python 3.10+ compatibility.

[Copilot]

Pull request overview

This PR drops Python 3.9 support and modernizes the codebase for Python 3.10+ compatibility. It updates type hints to use PEP 604 union syntax (X | Y instead of Optional[X]/Union[X, Y]), bumps CI and documentation references, and regenerates poetry.lock to remove Python 3.9-specific dependency entries.

Changes:

• Replaced all Optional[X] and Union[X, Y] type hints with PEP 604 syntax (X | None, X | Y) across all datatype, model, and login modules, removing now-unused typing imports.
• Updated CI workflows, pyproject.toml, README, and copilot instructions to reflect the new Python version range (3.10–3.14 stable, 3.15-dev).
• Regenerated poetry.lock to remove duplicate Python 3.9-era package entries and version markers.

Reviewed changes

Copilot reviewed 26 out of 27 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pyproject.toml	Removed Python 3.9 classifier, added 3.15, bumped minimum to ^3.10
poetry.lock	Removed Python 3.9 package variants and markers
README.md	Updated supported version text from 3.9–3.13 to 3.10–3.14
.github/workflows/python-pytest.yaml	Updated test matrix: dropped 3.9, added 3.14 stable and 3.15-dev
.github/workflows/python-lint.yaml	Bumped lint Python from 3.13 to 3.14
.github/workflows/publish-to-pypi.yaml	Bumped publish Python from 3.13 to 3.14
.github/copilot-instructions.md	Updated version references to 3.10+/3.15-dev
wikibaseintegrator/wbi_config.py	Replaced Union with | syntax, removed typing import
wikibaseintegrator/wbi_login.py	Replaced Optional with | None syntax
wikibaseintegrator/wbi_enums.py	Added blank line formatting fix
wikibaseintegrator/wbi_backoff.py	Added blank line formatting fixes
wikibaseintegrator/models/claims.py	Moved Callable import from typing to collections.abc
wikibaseintegrator/datatypes/*.py (13 files)	Replaced Optional/Union with PEP 604 syntax in all datatype modules

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