Why do we force the lazy python?

[jgart]

Hi Guixers,

Why do we check if python is a promise and then "force" it?

I'm just trying to understand the thinking for doing that since I didn't
write this code.

```
(let ((python (if (promise? python)
                  (force python)
                  python)))
     ...
```

I understand that force makes a promise evaluate (laziness). That's
about as much as I know.

guix/build-system/python.scm:119

all best,

jgart