Xiaomi Mijia thermometer & hygrometer in Home Assistant (LYWSD03MMC) on Hass.io

Connecting a thermometer & hygrometer (humiditymeter?) to Home Assistant can be a bit of a hassle. Who doesn’t want one in every room? There are lots of ways to get them set up, but it’s easy to make things nicely-complicated & expensive. Who doesn’t love spending more money than you need? I picked up a pack of Xiaomi Mijia sensors (or LYWSD03MMC, when you get to know them), they run on a coin-cell battery for about a year, have BTLE / Bluetooth Low Energy transmission of their status, and are pretty cheap.