Make ToPyObject impl for HashSet accept non-default hashers.

[ohadravid]

I had some code like

let s = HashSet<u32>::new();
s.to_object(py)

I switched to HashSet<_, FxBuildHasher>, and it stopped working.
So this PR changes the ToPyObject impl for HashSet to accept non-default hashers.

I didn't add a test since I saw that the other converter impls (for example HashMap, into_py) don't have tests for non-default BuildHashers, but let me know if you want me to add them anyway :)

[davidhewitt]

Thanks for the fix! As this is just adding a generic parameter I'm fine to skip adding a test.