So now I'm into a new failure mode, where again I thought I'd cribbed the pytest structure basically exactly:

File "/home/patrick/code/pants/src/python/pants/engine/target.py", line 839, in create
    address=tgt.address, **_get_field_set_fields_from_target(cls, tgt)
TypeError: __init__() missing 1 required positional argument: 'sources'