[3.15] gh-149162: Use _Py_LOCK_DONT_DETACH in intern_common (gh-149164) (gh-153814)

Avoids potential deadlocks with C++ static local initialization in
extensions that call PyUnicode_InternFromString during one-time
initialization.

(cherry picked from commit f62050d)

Co-authored-by: Sam Gross <colesbury@gmail.com>

Author: miss-islington

Misc/NEWS.d/next/Core_and_Builtins/2026-04-29-15-10-59.gh-issue-149162.BPPyrq.rst

@@ -0,0 +1,3 @@
+Fix a potential deadlock in :c:func:`PyUnicode_InternFromString` and other
+interning functions in the :term:`free-threaded build` when called from C++
+static local initializers.

Objects/unicodeobject.c

@@ -14344,7 +14344,7 @@ intern_common(PyInterpreterState *interp, PyObject *s /* stolen */,
     }
 #endif
 
-    FT_MUTEX_LOCK(INTERN_MUTEX);
+    FT_MUTEX_LOCK_FLAGS(INTERN_MUTEX, _Py_LOCK_DONT_DETACH);
     PyObject *t;
     {
         int res = PyDict_SetDefaultRef(interned, s, s, &t);