Improve lib2to3 future_features annotation

[giri256]

Part of #14031.

RefactoringTool populates Node.future_features from _detect_future_features, which collects tokenizer NAME values and returns a frozenset. This narrows the placeholder element type from Incomplete to str and removes the unused import.

Validation:

• focused tests/runtests.py stdlib/lib2to3: pre-commit, structure, strict Pyright, ty, mypy, and both regression suites passed
• direct detector probe returned frozenset({'annotations', 'generator_stop'}) with str elements
• global and module-specific stubtest failures reproduced identically on untouched origin/main; none reference future_features

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉