It's `search.versions_behind()`:
```
File "/opt/spacedock/KerbalStuff/search.py", line 50, in versions_behind
  compat = version.Version(mod.default_version.gameversion.friendly_version)
File "/usr/local/lib/python3.7/site-packages/packaging/version.py", line 277, in __init__
  raise InvalidVersion("Invalid version: '{0}'".format(version))
```