osbuild: pytoml is deprecated, replace with toml

Pytoml is no longer being maintained: https://github.com/avakar/pytoml
The author suggest switching to toml.

We already use the
```
try:
    import toml
except ModuleNotFoundError:
    import pytoml as toml
```
pattern in stages/org.osbuild.containers.storage.conf so use it in the tests too to prefer "toml" instead of pytoml.

tox.ini

@@ -18,7 +18,7 @@ deps =
     mako
     iniparse
     pyyaml
-    pytoml
+    toml
     pykickstart
     # required by pykickstart but not pulled in automatically :/
     requests