Pass the copy of existing environments to the subprocesses

Unset PYTHONPATH in virtual environments as tests running there
only need to see that env's paths in sys.path.

setuptools/tests/environment.py

@@ -18,6 +18,13 @@ class VirtualEnv(jaraco.envs.VirtualEnv):
     def run(self, cmd, *args, **kwargs):
         cmd = [self.exe(cmd[0])] + cmd[1:]
         kwargs = {"cwd": self.root, **kwargs}  # Allow overriding
+        # If no environment was passed as an argument
+        # create a copy of the current env to use when spawning a subprocess
+        # Unset PYTHONPATH - tests in venv need only that venv's paths in sys.path
+        env = kwargs.get("env") or dict(os.environ)
+        if "PYTHONPATH" in env:
+            del env["PYTHONPATH"]
+        kwargs["env"] = env
         return subprocess.check_output(cmd, *args, **kwargs)
 
 

setuptools/tests/fixtures.py

@@ -98,7 +98,10 @@ def venv(tmp_path, setuptools_wheel):
     env = environment.VirtualEnv()
     env.root = path.Path(tmp_path / 'venv')
     env.req = str(setuptools_wheel)
-    return env.create()
+    # Don't pass PYTHONPATH from the current environment to the created venv
+    # It only needs its own paths in sys.path
+    with contexts.environment(PYTHONPATH=None):
+        return env.create()
 
 
 @pytest.fixture