We set PYTHONPATH during testing.

--- coverage-7.10.4/tests/coveragetest.py.orig
+++ coverage-7.10.4/tests/coveragetest.py
@@ -475,11 +475,9 @@
 
     def add_test_modules_to_pythonpath(self) -> None:
         """Add our test modules directory to PYTHONPATH."""
-        # Check that there isn't already a PYTHONPATH.
-        assert os.getenv("PYTHONPATH") is None
         testmods = nice_file(self.working_root(), "tests/modules")
         zipfile = nice_file(self.working_root(), "tests/zipmods.zip")
-        self.set_environ("PYTHONPATH", testmods + os.pathsep + zipfile)
+        self.set_environ("PYTHONPATH", os.getenv("PYTHONPATH") + os.pathsep + testmods + os.pathsep + zipfile)
 
     def working_root(self) -> str:
         """Where is the root of the coverage.py working tree?"""