gh-113993: Don't immortalize in PyUnicode_InternInPlace; keep immortalizing in other API

[encukou]

• Switch PyUnicode_InternInPlace to _PyUnicode_InternMortal
• Keep immortalizing in functions that take char*, and are almost exclusively used with strings known at compile-time:
  • PyUnicode_InternFromString
  • PyDict_SetItemString
  • PyObject_SetAttrString
  • PyObject_DelAttrString
  • PyUnicode_InternFromString
  • PyModule_Add* convenience functions.
• Clarify docs
• Don't immortalize PyCSimpleType descriptor names, which aren't known at compile time

• Issue: gh-113993

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