make clickhouse-cityhash dependency optional

[samuelcolvin]

As per https://github.com/xzkostyan/clickhouse-cityhash/pull/3 clickhouse-cityhash can't currently be installed on M1 macs, it's also not required in many scenariois.

The solution is to only import it when it needs to be used, and make it an optional dependency.

I've installed this branch locally and it's working fine.

I'm not at all clear on how to correctly implement optional dependencies with poetry, please let me know if something needs changing here.

[long2ice]

Thanks! Please fix the CI

[samuelcolvin]

done 🤞, maybe could allow my workflows to run without awaiting approval?

[long2ice]

Which is just for first time contributor. Please fix the test, thanks!

[samuelcolvin]

@long2ice any chance you could approve the workflow?

[long2ice]

Thanks!