docs: import sys in the captured output example

AUTHORS

@@ -211,6 +211,7 @@ Henk-Jaap Wagenaar
 Henry Schreiner
 Holger Kohr
 Hugo van Kemenade
+Hugohong258
 Hui Wang (coldnight)
 Ian Bicking
 Ian Lesperance

doc/en/how-to/capture-stdout-stderr.rst

@@ -121,6 +121,10 @@ Here is an example test function that performs some output related checks:
 
 .. code-block:: python
 
+    # content of test_output.py
+    import sys
+
+
     def test_myoutput(capsys):  # or use "capfd" for fd-level
         print("hello")
         sys.stderr.write("world\n")
@@ -131,6 +135,12 @@ Here is an example test function that performs some output related checks:
         captured = capsys.readouterr()
         assert captured.out == "next\n"
 
+.. code-block:: pytest
+
+    $ pytest -q test_output.py
+    .                                                                    [100%]
+    1 passed in 0.12s
+
 The ``readouterr()`` call snapshots the output so far -
 and capturing will be continued.  After the test
 function finishes the original streams will