pywatchman raises SystemError on Python 3.10

The latest version of pywatchman (1.4.1) is not working on Python 3.10.

A `SystemError` occurs which can be reproduced as follows:

```python
import pywatchman
client = pywatchman.client()
client.capabilityCheck()
```

This code will trigger `SystemError: PY_SSIZE_T_CLEAN macro must be defined for '#' formats`.