scan_results() is returning bssids with an addiotional ":" on windows

```
>>> #tested on windows 10
>>> import pywifi
>>> wifi = pywifi.PyWiFi()
>>> iface = wifi.interfaces()[0]
>>> iface
<pywifi.iface.Interface object at 0x02F4D670>
>>> iface.scan()
>>> iface.scan_results()[0].bssid
'xx:xx:xx:xx:12:e3:'
```
Bssids should not end with an ":"