Treat `NoneType` fields as `None`/missing during deserialization of mappings

[tamasfe]

MCVE:

Rust side:

#[derive(Deserialize)]
struct Foo {
  #[serde(default)]
  foo: bool,
}

python side:

foo = {
  "foo": None,
}

depythonize::<Foo>(...) will fail with unexpected type: 'NoneType' object cannot be converted to '...'. However {} as input will work.

As far as I've looked, a check might be missing around here:

pythonize/src/de.rs

Line 450 in 4491cdb

seed.deserialize(&mut Depythonizer::from_object(&item))

If that's the case I can look into it open PR with the fix.