Fix nox configuration for PyPy

noxfile.py

@@ -9,7 +9,7 @@
 nox.options.reuse_existing_virtualenvs = True
 
 
-@nox.session(python=["2.7", "3.4", "3.5", "3.6", "3.7", "3.8", "pypy", "pypy3"])
+@nox.session(python=["2.7", "3.4", "3.5", "3.6", "3.7", "3.8", "pypy", "pypy3"],)
 def tests(session):
     def coverage(*args):
         session.run("python", "-m", "coverage", *args)
@@ -21,7 +21,17 @@ def coverage(*args):
         coverage("report", "-m", "--fail-under", "100")
     else:
         # Don't do coverage tracking for PyPy, since it's SLOW.
-        session.run("pytest", "--capture=no", "--strict", *session.posargs)
+        # Force PYTHONPATH to include the project directory first,
+        # (which is done automatically by python3 but not PyPY), to
+        # avoid testing the version of packaging that gets installed
+        # in site-packages as a dependency of pytest.
+        session.run(
+            "pytest",
+            "--capture=no",
+            "--strict",
+            *session.posargs,
+            env={"PYTHONPATH": "."},
+        )
 
 
 @nox.session(python="3.8")