Fix redundant import warning

[taiki-e]

error: the item `AsyncRead` is imported redundantly
   --> src/lib.rs:105:18
    |
105 | use futures_io::{AsyncRead, AsyncSeek, AsyncWrite};
    |                  ^^^^^^^^^
106 | use futures_lite::{future, prelude::*, ready};
    |                            ---------- the item `AsyncRead` is already imported here
    |
    = note: `-D unused-imports` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unused_imports)]`

error: the item `AsyncSeek` is imported redundantly
   --> src/lib.rs:105:29
    |
105 | use futures_io::{AsyncRead, AsyncSeek, AsyncWrite};
    |                             ^^^^^^^^^
106 | use futures_lite::{future, prelude::*, ready};
    |                            ---------- the item `AsyncSeek` is already imported here

error: the item `AsyncWrite` is imported redundantly
   --> src/lib.rs:105:40
    |
105 | use futures_io::{AsyncRead, AsyncSeek, AsyncWrite};
    |                                        ^^^^^^^^^^
106 | use futures_lite::{future, prelude::*, ready};
    |                            ---------- the item `AsyncWrite` is already imported here

[taiki-e]

Oh, Cargo mixes dev-deps...

https://github.com/smol-rs/blocking/actions/runs/8130067781/job/22217900369?pr=57

error[E0405]: cannot find trait `AsyncRead` in this scope
   --> src/lib.rs:769:32
    |
769 | impl<T: Read + Send + 'static> AsyncRead for Unblock<T> {
    |                                ^^^^^^^^^ not found in this scope
    |
help: consider importing this trait
    |
87  + use futures_io::AsyncRead;
    |

error[E0[40](https://github.com/smol-rs/blocking/actions/runs/8130067781/job/22217900369?pr=57#step:6:41)5]: cannot find trait `AsyncWrite` in this scope
   --> src/lib.rs:837:33
    |
837 | impl<T: Write + Send + 'static> AsyncWrite for Unblock<T> {
    |                                 ^^^^^^^^^^ not found in this scope
    |
help: consider importing this trait
    |
87  + use futures_io::AsyncWrite;
    |

error[E0405]: cannot find trait `AsyncSeek` in this scope
   --> src/lib.rs:920:32
    |
920 | impl<T: Seek + Send + 'static> AsyncSeek for Unblock<T> {
    |                                ^^^^^^^^^ not found in this scope
    |
help: consider importing this trait
    |
87  + use futures_io::AsyncSeek;
    |

[notgull]

Thanks!