Narrow tuples of singletons

[hauntsaninja]

Fixes #21598

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

CPython (Argument Clinic) (https://github.com/python/cpython)
+ Tools/clinic/libclinic/converter.py:216: error: Statement is unreachable  [unreachable]

[hauntsaninja]

Primer is a false positive. Here is a small repro:

def f(default: object, default_type: type[object] | tuple[type[object], ...]) -> None:
    reveal_type(default_type)
    if default_type == ():
        raise
    reveal_type(default_type)