doc: import pytest in conftest.py example in doctest.rst

[MLopez-Ibanez]

• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits. Please uncheck this if you prefer to squash the commits yourself.

Change is trivial or a small documentation fix (e.g., a typo or reword of a small section).

The conftest.py example does not work without the import pytest, which leads to the following error:

py310: commands[0]> pytest --doctest-modules --doctest-continue-on-failure --pyargs src
ImportError while loading conftest '[...]/mooplot/python/src/conftest.py'.
src/conftest.py:4: in <module>
    @pytest.fixture(autouse=True)
E   NameError: name 'pytest' is not defined

[nicoddemus]

Thanks!