gh-157242: Add byteswriter_check_consistency() #157723

Add byteswriter_check_consistency() to check that the bytes/bytearray object can be mutated. It is called by all PyBytesWriter functions.

• Add test_thread to PyBytesWriter tests.


• _PyBytes_IsMutable() now checks _PyObject_IsUniquelyReferenced().

• Issue: gh-157242
  <!-- /gh-issue-number -->