Support "async" validation functions

[barrywhart]

Currently, it appears that asynchronous validation functions are not supported, resulting in an error, "TypeError: Type is not JSON serializable: coroutine". This would be a useful feature. I have a case where an input (text) field needs to be validated for uniqueness, and this requires querying the application database, which is an async call.

[falkoschindler]

Hi @barrywhart,

Adding support for async validation functions might be a pretty handy feature. It's a bit tricky to fit into the existing API, but I just created a draft PR #4024.

[falkoschindler]

This feature has been released in version 2.7.0. ✔️

[barrywhart]

Great, thanks for the quick turnaround! I'll update my existing code to use this. I was using a hacky workaround before.