Moved from Pipenv to Poetry

All the Blender Institute Python webprojects are moving to Poetry; it works
better than Pipenv in various regards.

tests/test_shaman_transfer.py

@@ -21,7 +21,7 @@ import pathlib
 
 import responses
 
-from abstract_test import AbstractBlendFileTest
+from tests.abstract_test import AbstractBlendFileTest
 from blender_asset_tracer.pack.shaman import transfer
 
 httpmock = responses.RequestsMock()