Mask.__hash__: Fall back to slow hash if xxhash not available
Mask.__hash__: Fall back to slow hash if xxhash not available

This fixes the unit tests in the "python-package" CI job, which only
installs our PyPI package without the optional dependencies.

On my laptop the slow hash takes 1.5ms vs. 220µs for xxhash.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester