Document using PYTEST_VERSION to detect if a code is running inside pytest (#12153)

Related to #9502

changelog/12153.doc.rst

@@ -0,0 +1 @@
+Documented using :envvar:`PYTEST_VERSION` to detect if code is running from within a pytest run.

doc/en/example/simple.rst

@@ -405,35 +405,20 @@ Detect if running from within a pytest run
 Usually it is a bad idea to make application code
 behave differently if called from a test.  But if you
 absolutely must find out if your application code is
-running from a test you can do something like this:
+running from a test you can do this:
 
 .. code-block:: python
 
-    # content of your_module.py
+    import os
 
 
-    _called_from_test = False
-
-.. code-block:: python
-
-    # content of conftest.py
-
-
-    def pytest_configure(config):
-        your_module._called_from_test = True
-
-and then check for the ``your_module._called_from_test`` flag:
-
-.. code-block:: python
-
-    if your_module._called_from_test:
-        # called from within a test run
+    if os.environ.get("PYTEST_VERSION") is not None:
+        # Things you want to to do if your code is called by pytest.
         ...
     else:
-        # called "normally"
+        # Things you want to to do if your code is not called by pytest.
         ...
 
-accordingly in your application.
 
 Adding info to test report header
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