Support unstable-less nightly #299

Job	Run time
Test Suite	4m 9s
Test Suite (--no-default-features)	4m 11s
Clippy (stable)	26s
Check (stable)	18s
Test Suite (--features track-caller)	4m 15s
Rustfmt (stable)	16s
Test Suite (--features auto-install)	4m 8s
Miri	1m 6s
Test Suite (ubuntu-latest)	40s
Test Suite (windows-latest)	1m 19s
Test Suite (stable)	36s
Test Suite (macOS-latest)	55s
Test Suite (stable, --no-default-features)	40s
Test Suite (stable, --features unstable track-caller)	32s
Test Suite (stable, --features unstable pyo3)	34s
Test Suite (stable, --features unstable auto-install)	33s
Test Suite (stable, --all-features)	37s
Test Suite (beta)	39s
Test Suite (beta, --no-default-features)	33s
Test Suite (beta, --features unstable track-caller)	30s
Test Suite (beta, --features unstable pyo3)	35s
Test Suite (beta, --features unstable auto-install)	31s
Test Suite (beta, --all-features)	35s
Test Suite (nightly)	37s
Test Suite (nightly, --no-default-features)	34s
Test Suite (nightly, --features unstable track-caller)	33s
Test Suite (nightly, --features unstable pyo3)	33s
Test Suite (nightly, --features unstable auto-install)	34s
Test Suite (nightly, --all-features)	45s
	32m 14s

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