C API: Soft deprecate functions modifying str objects (PyUnicodeObject)

Python 3.14 added PyUnicodeWriter C API which is a safe way to create a Unicode object. IMO it's now time to soft deprecate the legacy way to create a str object using PyUnicode_New(): deprecate functions like PyUnicode_CopyCharacters() and PyUnicode_Resize().

The PyUnicode_New() API is more fragile. It's possible that a str object is "used" before it's fully initialized, which make following functions (modifying the str) fail or crash. It can also be exposed in Python by mistake before it's fully initialized.

PyUnicodeWriter has a different design than PyUnicode_New(): it's designed to append multiple strings, whereas PyUnicode_New() gives a direct access to an array of characters in a specific format (Py_UCS1, Py_UCS2 or Py_UCS4: 1, 2 or 4 bytes per character).

If PyUnicodeWriter doesn't fit well with existing code, it's possible to allocate an array of characters, write into this array, and then call PyUnicode_FromKindAndData() to create a str object.

Another option is to create a UTF-8 encode string and then call PyUnicode_FromString().

Linked PRs

• gh-157711
• gh-157712
• gh-157715

首先审查关联的 PR gh-157711、gh-157712 和 gh-157715，然后检查 C API 入口 PyUnicode_New()、PyUnicode_CopyCharacters() 和 PyUnicode_Resize()。就哪些 legacy 函数接受 soft-deprecation 处理达成一致，并记录或测试由此产生的 API 行为，即视为完成。