Injectable DLLs in Python? No, not afaik. External ESP for CA? Probably not possible, there's no current working Windowed Mode hack for CA and it doesn't have to necessarily be in Python anyway.
As for just standard memory editing, sure why not but HackShield prevents obtaining a valid process handle via OpenProcess by hooking into the API using it's driver aka, anti-cheat rootkit which is why you'd have to 'write OpenProcess from scratch'.
EDIT - ExternalESP may be possible, cause found a hack by Jeff which has windowed mode. Involves injecting though, but it could be made undetected by scrambling on inject. :3