gh-158803: Fix crash in bytes.join() on a concurrently mutated list (#158910)

In the free-threaded build, bytes.join() and bytearray.join() read
items from the list with borrowed references and without holding its
lock, so another thread could replace and free an item before it was
increfed.

Run the join under Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST, as
PyUnicode_Join() already does.