The recent work on extensible condition support in rbintegrations

introduced a backwards-compatible implementation for earlier versions of

Review Board, but this ended up introducing an import that wasn't

present in that module on older versions.
That import was only needed for type hints anyway, so it's been moved

out into a TYPE_CHECKING block and updated to use the correct module

and class for the supported releases.

Tested on Review Board 7.0.2, and verified I no longer hit import errors.