Update requires-python from >=3.8 to >=3.10

[mr-raj12]

PyQt6 requires Python >=3.10 and CI only tests 3.10+. Also update tox env_list to match CI coverage and add Python version classifiers.

Closes #2380

Description

Update Python version metadata in pyproject.toml and clean up stale references to Python 3.8/3.9 across project
configuration.

Changes across 2 files:

File	Change
pyproject.toml	requires-python: >=3.8 → >=3.10
pyproject.toml	Add explicit Python version classifiers (3.10, 3.11, 3.12, 3.13)
pyproject.toml	tox env_list: remove py38/py39, add py313
.github/scripts/generate-matrix.sh	Remove stale exclude rule referencing Python 3.8 + Borg 2.0.0b12
uv.lock	Regenerated to reflect new requires-python constraint

Related Issue

Closes #2380

Motivation and Context

The declared requires-python = ">=3.8" is misleading — PyQt6 (a core dependency) requires Python >=3.10, CI only
tests 3.10+, and Python 3.8 is EOL. Users on Python 3.8/3.9 who install Vorta hit confusing PyQt6 errors instead of
a clear version mismatch from pip. The tox env_list also listed py38/py39 but was missing py313, which is
already tested in CI.

How Has This Been Tested?

• uv sync — validates pyproject.toml parses correctly and dependencies resolve
• pre-commit run --all-files — all 9 checks passed (ruff, ruff-format, whitespace, etc.)
• CI matrix JSON validated — generate-matrix.sh output is valid JSON
• These are metadata-only changes with no functional code impact

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the
Developer Certificate of Origin.