print reactive expression: TypeError: __str__ returned non-string (type rx)

I'm on the latest `main` branch of param. When trying to print a reactive expression it raises an exception

```python
import param
rx_value = param.rx(10)
print(rx_value)
```

```bash
Traceback (most recent call last):
File "/home/jovyan/repos/private/param/script.py", line 3, in
print(rx_value)
TypeError: __str__ returned non-string (type rx)
```

The workaround is to print `rx_value.rx.value` instead. But this would be confusing to a lot of users. Please fix.