gh-121359: Run test_pyrepl in isolated mode (#121414)

run_repl() now pass the -I option (isolated mode) to Python if the
'env' parameter is not set.

Lib/test/test_pyrepl/test_pyrepl.py

@@ -927,8 +927,11 @@ def test_not_wiping_history_file(self):
 
     def run_repl(self, repl_input: str | list[str], env: dict | None = None) -> tuple[str, int]:
         master_fd, slave_fd = pty.openpty()
+        cmd = [sys.executable, "-i", "-u"]
+        if env is None:
+            cmd.append("-I")
         process = subprocess.Popen(
-            [sys.executable, "-i", "-u"],
+            cmd,
             stdin=slave_fd,
             stdout=slave_fd,
             stderr=slave_fd,