In this case, asynchronous method calls will return NULL themselves.

Since Python 3.9a4 , Py_REFCNT and Py_TYPE should no longer be used to
assign a value. Instead, Py_SET_REFCNT and Py_SET_TYPE should be used.

Since Python 3.10, Py_REFCNT can no longer be used to assign a value
at all.

See https://bugs.python.org/issue39573

This completes __Pyx_SET_REFCNT and __Pyx_SET_SIZE.
See https://bugs.python.org/issue39573

This also remove the associated mutex from the CyLock class.

This commit introduces the following changes:
- 'locked' becomes 'lock'
- 'locked&' becomes 'locked'
- references reachable from a locked reference are noted as 'lock->'