Add fallback implementation of PyCriticalSection_BeginMutex for Python 3.13t

include/pybind11/detail/internals.h

@@ -239,13 +239,24 @@ class pymutex {
     void unlock() { PyMutex_Unlock(&mutex); }
 };
 
+#    if !(defined(PY_VERSION_HEX) && PY_VERSION_HEX >= 0x030E00C1 /* 3.14.0rc1 */)
+// Forward declaration of internal slow path function for usage in pycritical_section
+void _PyCriticalSection_BeginSlow(PyCriticalSection *c, PyMutex *m);
+#    endif
+
 class pycritical_section {
     pymutex &mutex;
     PyCriticalSection cs;
 
 public:
     explicit pycritical_section(pymutex &m) : mutex(m) {
+        // PyCriticalSection_BeginMutex was added in Python 3.15.0a1 and backported to 3.14.0rc1
+#    if defined(PY_VERSION_HEX) && PY_VERSION_HEX >= 0x030E00C1 /* 3.14.0rc1 */
         PyCriticalSection_BeginMutex(&cs, &mutex.mutex);
+#    else
+        // Use the slow path of internal API `_PyCriticalSection_BeginMutex` for older versions
+        _PyCriticalSection_BeginSlow(&cs, &mutex.mutex);
+#    endif
     }
     ~pycritical_section() { PyCriticalSection_End(&cs); }