Not return result in as_array & as_slice

[kngwyu]

1. If we construct PyArray via Rust API
   It's already type checked.
2. If we construct PyArray in Python
   It's type checked when FromPyObject::extract.

In both cases PyArray is already type checked when we can apply methods to it.
So I think we don't have to do type check in as_array and as_slice, though it's left as assertion.

[kngwyu]

Now CI fails due to rust-lang/rust#55380