[CI] Fix pytest imports

Author: stephane-caron

tox.ini

@@ -61,7 +61,7 @@ deps =
     robomeshcat-py{38,39}: robomeshcat >= 1.0.3
     yourdfpy: yourdfpy >=0.0.52
 commands =
-    pytest tests/loaders/test_{env:LOADER}.py
+    pytest --import-mode=importlib tests/loaders/test_{env:LOADER}.py
 
 [flake8]
 max-line-length = 88