Add support for PEP 661 (sentinels)

[JelleZijlstra]

See PEP 661 and python/typing#2277

[sobolevn]

Great stuff! 👍

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[cdce8p]

Thanks for working on it! I took some time to start testing it for Home Assistant.

I did encounter an issue during the cache restore

Traceback (most recent call last):
  File "/.../python3.14/site-packages/mypy/__main__.py", line 16, in console_entry
    main()
    ~~~~^^
  File "mypy/main.py", line 154, in main
  File "mypy/main.py", line 244, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "mypy/build.py", line 422, in build
  File "mypy/build.py", line 537, in build_inner
    graph = dispatch(sources, manager, stdout, connect_threads)
  File "mypy/build.py", line 4150, in dispatch
    process_graph(graph, manager)
  File "mypy/build.py", line 4618, in process_graph
    done, still_working, results = manager.wait_for_done(graph)
  File "mypy/build.py", line 1492, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "mypy/build.py", line 4765, in process_stale_scc
    maybe_load_deps(graph, ascc, manager)
  File "mypy/build.py", line 4749, in maybe_load_deps
    process_fresh_modules(graph, sorted(prev_scc.mod_ids), manager)
  File "mypy/build.py", line 4700, in process_fresh_modules
    graph[id].load_tree()
  File "mypy/build.py", line 3120, in load_tree
    self.tree = MypyFile.read(data_io)
  File "mypy/nodes.py", line 609, in read
  File "mypy/nodes.py", line 5131, in read
    [(read_str_bare(data), SymbolTableNode.read(data)) for _ in range(size)]
  File "mypy/nodes.py", line 5046, in read
    sym._node_bytes = extract_symbol(data)
ValueError: Unsupported tag: 8

Looks like LITERAL_SENTINEL needs to be added to librt_internal.c as well from what I can tell.

--
Besides that I did see a couple of Cannot assign to final name messages which were unexpected. I've defined most sentinels in the style _SENTINEL = Sentinel("_SENTINEL"). I'd have expected that as long as the sentinel isn't imported, I can reuse the value / variable name just fine in a different module. Am I missing something?