Fix importing unitxt on Python 3.14 and 3.15

[cristianchiriac]

import unitxt fails on Python 3.14 and 3.15, and CI doesn't catch it because it only runs older versions.

Python 3.14 (PEP 649/749): a class body no longer puts __annotations__ in the class namespace, only an __annotate__ function. get_fields() read attrs.get("__annotations__", {}), so no fields were found. The first Dataclass built with keyword arguments then raised at import:

unitxt.dataclass.UnexpectedArgumentError: Unexpected keyword argument(s) {'field': 'prediction', 'to_field': 'raw_prediction'} for class RecursiveCopy.
Should be one of: []

• get_fields() now uses _namespace_annotations(attrs). It keeps __annotations__ when the namespace has it (all versions before 3.14), and otherwise uses annotationlib.get_annotate_from_class_namespace() + call_annotate_function(..., Format.FORWARDREF), the documented way for metaclasses.
• Dataclass.to_dict(classes=...) read cls.__annotations__, which no longer resolves through an instance on 3.14. It now uses inspect.get_annotations() on the class (falling back to the class __dict__ before 3.10).

Python 3.15:

• dataclasses._MISSING_TYPE is gone (MISSING became a PEP 661 sentinel), so update_init_signature compares with dataclasses.MISSING.
• typing._eval_type() now requires type_params, so eval_forward_ref passes type_params=() on 3.13+. (3.13/3.14 accept it.)

Tests:

• test_fields_from_annotations: fields of a base and a child class are collected, and keyword init works.
• test_eval_forward_ref: plain and custom forward refs.

Results for pytest tests/library/test_dataclass.py tests/library/test_type_utils.py (pip install -e ., Docker python:<v>):

Python	before	after
3.9		59 passed
3.12	1 failed (test_parse_malformed_string)	same 1 failed, 58 passed
3.14	import unitxt fails	5 failed, 54 passed
3.15	import unitxt fails	5 failed, 54 passed

test_parse_malformed_string already fails on main with 3.12. The 4 other failures on 3.14/3.15 (test_parse_union_type, test_format_type_string, test_parse_type_string_with_literal, test_to_string_nested_generic_types) are in type-string parsing: on 3.14, safe_eval of union types raises KeyError: '__import__' with the empty __builtins__. That needs its own change, so I left it out of this PR.

ruff / ruff format (v0.1.6, as in pre-commit) and codespell are clean. Commits are signed off (DCO).