hotplugging usb device not recognized with enumerate_devices()

If a usb device is plugged in after `import frida`, the device won't show up in frida.enumerate_devices(). It's reproducible with the interactive shell.

A workaround is to reload frida every time before getting the devices which seem to be a dirty hack.
```
import importlib
importlib.reload(frida)
devices = frida.enumerate_devices()
```