[8.1.x] doc: `import pytest` in `conftest.py` example in `doctest.rst` (#12182)

Co-authored-by: Manuel López-Ibáñez <2620021+MLopez-Ibanez@users.noreply.github.com>

doc/en/how-to/doctest.rst

@@ -224,6 +224,7 @@ place the objects you want to appear in the doctest namespace:
 .. code-block:: python
 
     # content of conftest.py
+    import pytest
     import numpy