doc: Update npm docs for debian sid

There is no python package anymore, you have to explicitly
pick which version of python to install and we need to inform
npm of the executable name.

Author: nhooyr

doc/npm.md

@@ -21,7 +21,9 @@ sudo apt-get install -y \
   pkg-config \
   libx11-dev \
   libxkbfile-dev \
-  libsecret-1-dev
+  libsecret-1-dev \
+  python3
+npm config set python python3
 ```
 
 ## Fedora, CentOS, RHEL