feat: Enforce Python 3.8+ requirement and add multi-version CI testing

[Iamrodos]

This PR aligns Python version requirements across documentation, packaging, and CI to prevent compatibility issues and make requirements explicit for contributors.

Changes

• setup.py: Add python_requires=">=3.8" to enforce minimum version at pip install time
• README.rst: Explicitly document "Python 3.8 or higher" requirement (was just "Python")
• CI (lint.yml): Add test matrix for Python 3.8, 3.9, 3.10, 3.11, 3.12, 3.13, and 3.14

Rationale

This project already declares Python 3.8-3.12 support via setup.py classifiers, but this requirement was not enforced or tested. This PR makes that declaration official and enforceable.

Why Python 3.8+ specifically:

1. Already declared: setup.py classifiers claim 3.8-3.12 support only
2. EOL status: Python 3.6 (EOL Dec 2021) and 3.7 (EOL June 2023) are no longer receiving security updates
3. Usage data: PyPI stats show ~99% of downloads are on Python 3.8+ (5,186/month)
   • Python 3.12: ~60-100+ downloads/day
   • Python 3.8: ~50-65 downloads/day
   • Python 3.6-3.7: <100/month total (likely CI/bots)
   • Python 2.7/3.5: effectively zero usage

Historical context:

• Before the attachment feature (PR feat: Add attachment download support for issues and pull requests #439), the codebase only used .format() strings and could technically support Python 3.0+
• The attachment feature introduced f-strings, raising the actual minimum to Python 3.6+
• Given we could now choose 3.6+, we align with the already-declared 3.8+ to match EOL policies and actual usage

The gap this fixes:

Without explicit enforcement and CI testing, contributors could easily introduce incompatible syntax:

Common Python features that require newer versions:

• Python 3.6+: f-strings (PEP 498) - f"hello {name}" - Already in codebase
• Python 3.8+: Walrus operator := (PEP 572) - if (n := len(a)) > 10:
• Python 3.9+: Dict merge operator | (PEP 584) - d1 | d2
• Python 3.9+: Type hints with [] (PEP 585) - list[int] instead of List[int]
• Python 3.10+: Union type syntax X | Y (PEP 604) - def foo(x: int | str):
• Python 3.10+: match/case statements (PEP 634) - Structural pattern matching

These features are increasingly common in modern Python code. Without CI testing on all supported versions, a PR using these features could pass code review and break users on older Python versions.

Impact

Benefits

• For users: pip blocks installation on unsupported Python versions, preventing runtime failures
• For contributors: README clearly documents minimum Python 3.8+ requirement upfront
• For maintainers: CI automatically catches incompatible syntax across all 7 supported versions (3.8-3.14)
• Net benefit: Protects 5,000+ monthly active users from future code incompatibilities

Drawbacks

• Breaking change: Users on Python <3.8 cannot install new versions (pip will error with clear message)
• Affected users: Minimal impact (~1-2 real users based on download stats, all on EOL Python versions)
• Migration path: Users on EOL Python versions can continue using current version (0.50.3) indefinitely

Testing

• All changes are declarative (no code logic changes)
• CI will automatically test on all 7 Python versions on this PR
• python_requires is a standard setuptools feature used by thousands of packages

Thank you for reviewing and considering this change.

[Iamrodos]

Closing this PR in favor of #441 - rebased onto clean upstream/master to exclude unrelated attachment feature commits. The Python version requirement changes are identical.