Support for from __future__ import annotations

[mosquito]

This pull request improves support for Python's from __future__ import annotations (PEP 563/649) in the argclass library by introducing a new utility function to robustly resolve type annotations, even when they are stringified. The changes ensure that type hints are correctly interpreted in modern Python versions and add comprehensive tests to verify this behavior.

Enhanced annotation resolution:

• Added a new resolve_annotations function in argclass/utils.py that uses typing.get_type_hints() to resolve stringified annotations, falling back to the legacy merge_annotations if needed. This function is now used in both argclass/parser.py and argclass/store.py to retrieve class annotations, replacing direct usage of merge_annotations. [1] [2] [3] [4] [5] [6]

Testing for future annotations:

• Added a new test file tests/test_future_annotations.py with comprehensive test cases to ensure argclass works correctly when from __future__ import annotations is enabled, covering basic types, optionals, literals, lists, enums, groups, subparsers, and secrets.