I wanted to install the rtl_443 package onto my Raspberry Pi, running Buster edition of Raspbian, so I could read the data from my WH1080. The GitHub page pointed me to the Raspbian testing repository.
To install this package from testing, the first task was to edit /etc/apt/sources.list and add the repository;
deb http://raspbian.raspberrypi.org/raspbian/ testing main
Then sudo apt update . At this point I then just did sudo apt-get install rtl-433 but I suggest you don’t if you don’t want everything updated from the testing repository (I didn’t).
Instead create file /etc/apt/preferences and include;
Pin: release a=stable
Pin: release o=Raspberry Pi Foundation,a=testing,n=buster,l=Raspberry Pi Foundation,c=main,b=armhf
Pin: release a=testing
Pin: release a=unstable
and sudo apt update
This will put the raspbian testing repository as a lower priority than the main and standard repositories. You can check this by using apt-cache policy to list the repositories.
Note, by default, Raspbian is picking up a Debian repository – no idea why, but it is a bit confusing if you do a apt list --upgradable
All that is needed to install the package is a sudo apt install rtl_433