Change the ST value from:
ssdp:all

to:
upnp:rootdevice

Such that:
SSDP_DICT = {'ip_address': '239.255.255.250',
'port': 1900,
'mx': 10,
'st': 'upnp:rootdevice'}

Old/previous value did not report any Wemo devices. Updated value now shows all UPNP devices.