Fix build on latest nightlies when using the `nightly` feature #77

Job	Run time
clippy	12s
test-doc	1m 3s
test-cross (ubuntu-latest, mips-unknown-linux-gnu, stable)	3s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	25s
test-cross (ubuntu-latest, arm-unknown-linux-gnueabi, stable)	48s
test-msrv	17s
test-no-std	14s
test (macos-latest, x86_64-apple-darwin, stable)	34s
test (windows-latest, x86_64-pc-windows-gnu, stable)	1m 2s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 53s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	59s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	37s
test-cross (ubuntu-latest, aarch64-unknown-linux-gnu, stable)	43s
test-cross (ubuntu-latest, arm-unknown-linux-gnueabi, nightly)	1m 46s
test-cross (ubuntu-latest, aarch64-unknown-linux-gnu, nightly)	1m 41s
	12m 17s

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