mypy

67ada309 - [stubtest] Check runtime availability of private types not marked `@type_check_only` (#19574)

[stubtest] Check runtime availability of private types not marked `@type_check_only` (#19574) Currently, stubtest ignores symbols that are 1) private and 2) not present at runtime. Private names are often used for stub-only constructs (which we don't want stubtest to complain about), but they can also denote actual private runtime types. Without other context, it's often ambiguous which category a private names falls into. For class definitions, we can make the intent explicit by decorating the class with `@type_check_only`. Typeshed already does this for the most part, but there's still quite a few cases of stub-only private types that aren't marked `@type_check_only` (especially in older stubs). Going forward, I think it could a good idea to make stubtest start checking for `@type_check_only` on private types that are not available at runtime. A few special cases are needed to avoid false positives on types that cannot be marked with `@type_check_only`, namely: * NewType declarations * TypedDicts with fields that are not valid identifiers (and therefore need to use the functional syntax) ~This PR creates quite a few new errors on typeshed (\~100 in stdlib, \~300 in third-party stubs), which we'll probably want to sift through before merging this.~ As of this writing, all typeshed hits have been fixed.