Since, as you point out, json.tool is made for convenience, I see the reverse of pretty-printing (minifying) being just as convenient:

$ cat > expanded.json <<EOF
> {
>     "foo": "bar",
>     "json": "obj"
> }
> EOF
$ ./python.exe -m json.tool --minify expanded.json minf.json
$ cat minf.json 
{"foo":"bar","json":"obj"}