rust-clippy: fix remaining warnings in `hg-cpython`

rust/hg-cpython/src/copy_tracing.rs

@@ -103,7 +103,7 b' pub fn combine_changeset_copies_wrapper('
                 // thread can drop it. Otherwise the GIL would be implicitly
                 // acquired here through `impl Drop for PyBytes`.
                 if let Some(bytes) = opt_bytes {
-                    if let Err(_) = pybytes_sender.send(bytes.unwrap()) {
+                    if pybytes_sender.send(bytes.unwrap()).is_err() {
                         // The channel is disconnected, meaning the parent
                         // thread panicked or returned
                         // early through

rust/hg-cpython/src/pybytes_deref.rs

@@ -47,6 +47,7 b' fn require_send<T: Send>() {}'
 
 #[allow(unused)]
 fn static_assert_pybytes_is_send() {
+    #[allow(clippy::no_effect)]
     require_send::<PyBytes>;
 }