bug#36900: key-mon crashes on launch

[Alexandros Theodotou]

Some things discovered on IRC:

"works for me with [patch below] and by running: $ ./pre-inst-env guix
environment --ad-hoc key-mon gdk-pixbuf+svg --pure -- $(which bash) -c
'DISPLAY=:0 key-mon'"

"figuring out where to propagate gdk-pixbuf+svg remains to be found"

```patch
diff --git a/gnu/packages/python-xyz.scm b/gnu/packages/python-xyz.scm
index 23e43b7e75..c418a9c7e0 100644
--- a/gnu/packages/python-xyz.scm
+++ b/gnu/packages/python-xyz.scm
@@ -8046,8 +8046,9 @@ falling into the Python interpreter.")
     (native-inputs
      `(("python-mock" ,python-mock)
        ("python-setuptools-scm" ,python-setuptools-scm)
-       ("python-six" ,python-six)
        ("xorg-server" ,xorg-server)))
+    (propagated-inputs
+     `(("python-six" ,python-six)))
     (home-page "https://github.com/python-xlib/python-xlib";)
     (synopsis "Python X11 client library")
     (description
```

FYI, the python-screenkey patch also has a similar issue, this could be
it.

Thanks,
Alex