Work around a Poetry bug

Poetry does not include the src directory in the sdist archive, if a parent
directory of the project also contains a Poetry project. This is the case
because pytest-datadir copies the example project to a temporary directory,
and nox_poetry sets TMPDIR to a location under the session directory, via
session.create_tmp().

noxfile.py

@@ -129,6 +129,7 @@ def tests(session: Session) -> None:
         session.install("dataclasses")
 
     try:
+        session.env.pop("TMPDIR", None)
         session.run("coverage", "run", "--parallel", "-m", "pytest", *session.posargs)
     finally:
         if session.interactive: