blackd has no API to get server version

[jacobbogdanov]

I am working on a client to blackd and would like to be able to get the version of the blackd server from the http endpoint.

This is for the same use-case that black itself has; disk caching of already formatted files. It would be advantageous to be able to check the cache and avoid the network IO.
Whether there is a new API endpoint, /version, or the response to each POST contains a header containing the server version should be equivalent for this particular use-case

Operating system: macOS
Python version: python3.6
Black version: blackd, version 19.3b0
Does also happen on master: as far as I can tell from the source code

[zsol]

I've also bumped into this while writing a blackd client, I'd be happy to review a PR :)

[linusg]

I'd be happy to review a PR :)

@zsol please see #1046 :)