The previous typing meant `threading.ExceptHookArgs` could not be used
to type a value.
The new typing follows what cpython does in the happy path (`_thread`
exists rather than the pure-python fallback being used).
Fixes#4767.
This makes mypy think that conditions like
parameter.kind is Parameter.POSITIONAL_OR_KEYWORD
are always false, which triggers `unreachable` warnings, among other
problems.
NodeVisitor recurses over an AST tree. When encountering a node, it
checks if a method called `visit_{node.__class__.__name__}` exists, and
calls it if so, otherwise calls the generic visitor.
Add the possible methods to NodeVisitor. This is not exactly correct,
since the methods don't *actually* exist on NodeVisitor, e.g.
`NodeVisitor().visit_Module(...)` doesn't work. But it's nice for
subclasses to know which methods they can override and which type they
should have.
Ran Benita