fixed problem with session attribute being incorrectly formatted which resulted in error for python 3.9+

Author: dbkeator

pyproject.toml

@@ -3,3 +3,4 @@ requires = ["setuptools >= 46.4.0", "versioningit"]
 build-backend = "setuptools.build_meta"
 
 [tool.versioningit]
+default-version = "0+unknown"

tests/experiment/test_experiment.py

@@ -52,7 +52,7 @@ def main():
 
     # test add session to graph and associate with project
     session = Session(project)
-    session.add_attributes({Constants.NIDM: "test"})
+    session.add_attributes({Constants.NIDM["test"]: "test"})
     # project.add_sessions(session)
 
     # test add MR acquisition activity / entity to graph and associate with session