> it seems LibRaw can be installed on an RPi

Why "seems", I checked with my RPi before posting.

Which method of installation to use depends on your needs.

Generally if you want a package (rawpy in your case), it's a good idea to follow the instructions accompanying the package, especially when those instructions refer to a particular (not the latest) version of a library. You will need to install git and cmake,

> sudo apt update
> sudo apt upgrade
> sudo apt install git
> sudo apt install cmake

After LibRaw is installed the way rawpi instructions suggest, check

> echo $LD_LIBRARY_PATH

If it's empty, and you want to use LibRaw independently / check the installation:

> LD_LIBRARY_PATH=/usr/local/lib
> export LD_LIBRARY_PATH