Fix set operations on frame flags

[shkyyy18]

Bug

Flags inherits the MutableSet operations, but its constructor expects flag definitions rather than the flag-name iterable that those operations supply. As a result, even frame.flags &= {"END_HEADERS"} raises AttributeError: 'str' object has no attribute 'name' when another flag must be removed. Non-mutating intersection, union, difference and symmetric difference fail in the same way, including reflected operations with a plain set.

Change

Override the set mixin's _from_iterable factory to return an ordinary set. That avoids treating string elements as Flag definitions. Non-mutating results do not carry frame-specific validation; in-place operations still retain the original Flags object and validate additions. This matches the regular-set behavior described by Flags and avoids changing the constructor or the frame's valid-flag definitions.

Add coverage for all four non-mutating operations in both operand orders, all four in-place operations, an empty result, and invalid additions. Include a changelog entry.

Validation

On Python 3.12 / Windows, using the repository's pinned public HTTP/2 fixture submodule:

• Baseline full suite: 148 passed, 100% branch coverage.
• Flags tests before the fix: 10 failed, 9 passed (nine AttributeError regressions and one empty-result type assertion).
• Full suite after the fix: 163 passed, 100% branch coverage.
• ruff check src/: passed.
• mypy src/ (strict): passed.
• Other Python/platform environments, docs and packaging checks were not run.

AI assistance was used to identify, implement, and test this fix. Tests use synthetic flags and the existing public fixture collection.