Fix unused_imports warning on latest nightly #802

Job	Run time
Test (ubuntu-20.04, stable)	2m 5s
Docker (aarch64-unknown-linux-gnu)	1m 15s
Test (ubuntu-20.04, beta)	3m 9s
Docker (arm-unknown-linux-gnueabihf)	1m 2s
iOS (aarch64-apple-ios, iphoneos)	36s
Build Targets (wasm32-unknown-unknown)	16s
Test (ubuntu-20.04, nightly)	2m 5s
Docker (armv7-unknown-linux-gnueabihf)	45s
iOS (x86_64-apple-ios, iphonesimulator)	43s
Build Targets (wasm32-wasi)	16s
Test (macos-latest, stable)	5m 8s
Docker (i586-unknown-linux-gnu)	1m 14s
Build Targets (x86_64-unknown-fuchsia)	16s
Test (macos-latest, nightly)	4m 15s
Docker (i686-unknown-linux-gnu)	48s
Build Targets (x86_64-fortanix-unknown-sgx)	26s
Test (windows-latest, stable-x86_64-msvc)	2m 20s
Miri	3m 45s
Docker (powerpc64-unknown-linux-gnu)	1m 26s
Build Targets (x86_64-unknown-illumos)	29s
Test (windows-latest, stable-i686-msvc)	3m 23s
Docker (s390x-unknown-linux-gnu)	1m 13s
Test (windows-latest, stable-x86_64-gnu)	2m 32s
Docker (x86_64-pc-windows-gnu)	36s
MSRV	3m 11s
Rustfmt	9s
Docker (x86_64-unknown-linux-gnu)	54s
Windows AArch64	1m 45s
Docker (x86_64-unknown-linux-musl)	45s
Docker (arm-linux-androideabi)	1m 51s
Docker (armv7-linux-androideabi)	1m 7s
Docker (aarch64-linux-android)	2m 1s
Docker (i686-linux-android)	1m 20s
Docker (x86_64-linux-android)	2m 10s
	55m 16s

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