Merge 'types.UnionType' with `typing._UnionGenericAlias`, not `typing.Union`

Bug report

I think #105499 was a mistake. types.UnionType was intentionally named so to avoid confusion with typing.Union (see #88895). types.UnionType should not be subscriptable because it is not generic type (and if it was a generic type, subscription would have different semantic than for typing.Union). types.UnionType corresponds to typing._UnionGenericAlias, not typing.Union (see https://github.com/python/cpython/issues/89581#issuecomment-1093932536).

So we should restore typing.Union, make typing._UnionGenericAlias an alias of types.UnionType, and make types.UnionType non-subscriptable again.

Linked PRs

• gh-137069
• gh-138967

先閱讀 issue 對 #105499、#88895 和 #89581 的引用，然後檢查 gh-137069 和 gh-138967 中連結的工作。追蹤 types.UnionType、typing.Union 和 typing._UnionGenericAlias 之間的關係；完成的標準是恢復預期的別名行為和不可進行下標操作的行為，並且相關測試通過。