Implement keccak256 operator and softfork #1964

Job	Run time
Checks	45s
Wheel on macos-13 py-3.8	2m 59s
Wheel on ubuntu-latest py-3.8	2m 46s
Wheel on windows-latest py-3.8	3m 26s
coverage	8m 14s
Run fuzz targets (macos-13, 3.8)	33m 57s
Unit tests (macos-13, 3.8)	8m 24s
Run fuzz targets (ubuntu-latest, 3.8)	14m 30s
Unit tests (ubuntu-latest, 3.8)	3m 42s
Unit tests (windows-latest, 3.8)	5m 40s
Upload to PyPI	7s
	1h 24m 30s

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