Fix serious reference counting issue in nb_error.h

This is a quite serious error affecting nanobind on Python versions 3.11
and earlier. The ``python_error::traceback()`` incorrectly stole a
reference, likely causing undefined behavior via use-after-free. This
function was called as part of ``python_error::what()``, which is a
frequently used routine that generates a readable error message +
backtrace on the C++ side.

include/nanobind/nb_error.h

@@ -65,7 +65,7 @@ class NB_EXPORT python_error : public std::exception {
 
 #if PY_VERSION_HEX < 0x030C0000
     handle type() const { return m_type; }
-    object traceback() const { return steal(m_traceback); }
+    object traceback() const { return borrow(m_traceback); }
 #else
     handle type() const { return value().type(); }
     object traceback() const { return steal(PyException_GetTraceback(m_value)); }