Add `_Py_ALIGN_AS` as per C API WG vote: https://github.com/capi-workgroup/decisions/issues/61
This patch only adds it to free-threaded builds; the `#ifdef Py_GIL_DISABLED`
can be removed in the future.
Use this to revert `PyASCIIObject` memory layout for non-free-threaded builds.
The long-term plan is to deprecate the entire struct; until that happens
it's better to keep it unchanged, as courtesy to people that rely on it despite
it not being stable ABI.
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