Pass importmode to import_path in DoctestModule

[alicederyn]

This allows doctest to be used with namespace modules in tox, as illustrated in the new unit test. See also #3396.

[asottile]

[asottile]

and just to confirm the failure mode of your test before the fix:

$ pytest testing/test_doctest.py
============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.2.0.dev179+g85000f037.d20220629, pluggy-1.0.0
rootdir: /tmp/pytest, configfile: pyproject.toml
plugins: hypothesis-6.48.1
collected 134 items                                                            

testing/test_doctest.py ....................F........................... [ 35%]
...........................x............................................ [ 89%]
..............                                                           [100%]

=================================== FAILURES ===================================
_________________________ TestDoctests.test_importmode _________________________

self = <test_doctest.TestDoctests object at 0x7f8fe2b40040>
pytester = <Pytester PosixPath('/tmp/pytest-of-asottile/pytest-10/test_importmode0')>

    def test_importmode(self, pytester: Pytester):
        p = pytester.makepyfile(
            **{
                "namespacepkg/innerpkg/__init__.py": "",
                "namespacepkg/innerpkg/a.py": """
                  def some_func():
                    return 42
                """,
                "namespacepkg/innerpkg/b.py": """
                  from namespacepkg.innerpkg.a import some_func
                  def my_func():
                    '''
                    >>> my_func()
                    42
                    '''
                    return some_func()
                """,
            }
        )
        reprec = pytester.inline_run(p, "--doctest-modules", "--import-mode=importlib")
>       reprec.assertoutcome(passed=1)
E       AssertionError: ([], [], [<CollectReport 'namespacepkg/innerpkg/b.py' lenresult=0 outcome='failed'>])
E       assert {'failed': 1,... 'skipped': 0} == {'failed': 0,... 'skipped': 0}
E         Omitting 1 identical items, use -vv to show
E         Differing items:
E         {'passed': 0} != {'passed': 1}
E         {'failed': 1} != {'failed': 0}
E         Use -v to get more diff

/tmp/pytest/testing/test_doctest.py:136: AssertionError
----------------------------- Captured stdout call -----------------------------
============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.2.0.dev179+g85000f037.d20220629, pluggy-1.0.0
rootdir: /tmp/pytest-of-asottile/pytest-10/test_importmode0
collected 0 items / 1 error

==================================== ERRORS ====================================
_________________ ERROR collecting namespacepkg/innerpkg/b.py __________________
namespacepkg/innerpkg/b.py:1: in <module>
    from namespacepkg.innerpkg.a import some_func
E   ModuleNotFoundError: No module named 'namespacepkg'
=========================== short test summary info ============================
ERROR namespacepkg/innerpkg/b.py - ModuleNotFoundError: No module named 'name...
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 0.09s ===============================
=========================== short test summary info ============================
FAILED testing/test_doctest.py::TestDoctests::test_importmode - AssertionError: ([], [], [<CollectReport 'namespacepkg/innerpkg/b.py' lenre...
=================== 1 failed, 132 passed, 1 xfailed in 6.41s ===================