Update to current Python (3.11)

[skyelaird]

Major improvements:

• Update minimum Python requirement from 3.8 to 3.11
• Modernize all type hints to use Python 3.11+ syntax:
  • Replace Dict[] with dict[]
  • Replace List[] with list[]
  • Replace Tuple[] with tuple[]
  • Replace Optional[T] with T | None
  • Remove typing imports where no longer needed
• Update CI/CD to test Python 3.11, 3.12, 3.13
• Add @cache decorator to make_sincos_array() for performance
• Return tuple instead of list from make_sincos_array() for immutability

Benefits:

• Cleaner, more readable type hints
• Better performance (cached trig calculations)
• Automatic 10-60% speedup from Python 3.11+ interpreter
• Modern best practices aligned with current Python standards

Files modified:

• pyproject.toml: Updated requires-python, classifiers, tool versions
• .github/workflows/validation.yml: Test Python 3.11-3.13
• All 13 source files: Modernized type hints throughout
• geomagnetic.py: Added @cache for performance