pytester: always close stdin pipe in pytester.run()

If the user passed stdin=PIPE for some reason, they have no way to close
it themselves since it is not exposed.

src/_pytest/pytester.py

@@ -1349,7 +1349,7 @@ class Pytester:
                 stderr=f2,
                 close_fds=(sys.platform != "win32"),
             )
-            if isinstance(stdin, bytes):
+            if popen.stdin is not None:
                 popen.stdin.close()
 
             def handle_timeout() -> None: