Support unstable-less nightly #298

Job	Run time
Miri	1m 4s
Rustfmt (stable)	12s
Clippy (stable)	26s
Check (stable)	17s
Test Suite	13s
Test Suite (--no-default-features)	11s
Test Suite (stable)	10s
Test Suite (ubuntu-latest)	35s
Test Suite (stable, --no-default-features)	1s
Test Suite (--features track-caller)	10s
Test Suite (windows-latest)	1m 16s
Test Suite (stable, --features track-caller)	1s
Test Suite (--features auto-install)	11s
Test Suite (macOS-latest)	54s
Test Suite (stable, --features pyo3)	12s
Test Suite (stable, --features auto-install)	13s
Test Suite (stable, --all-features)	11s
Test Suite (beta)	10s
Test Suite (beta, --no-default-features)	10s
Test Suite (beta, --features track-caller)	11s
Test Suite (beta, --features pyo3)	10s
Test Suite (beta, --features auto-install)	11s
Test Suite (beta, --all-features)	13s
Test Suite (nightly)	13s
Test Suite (nightly, --no-default-features)	10s
Test Suite (nightly, --features track-caller)	12s
Test Suite (nightly, --features pyo3)	12s
Test Suite (nightly, --features auto-install)	11s
Test Suite (nightly, --all-features)	11s
	8m 31s

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