Make compatible with pytest-asyncio

`pytest-asyncio` reads [1] `collector.obj` during test collection which
raises an `ImportError` for `DocTestTextfilePlus`. We fix this by
aligning that class with pytest’s `DoctestTextfile` [2] and setting
`obj` to `None`.

Fixes #256

[1]: https://github.com/pytest-dev/pytest-asyncio/blob/eb63d5ad0ca21041726ada3d5c00211d36418a9b/pytest_asyncio/plugin.py#L640
[2]: https://github.com/pytest-dev/pytest/blob/fc56ae365fcdea800f91683186136a8191e22399/src/_pytest/doctest.py#L421

Author: geigerzaehler

.github/workflows/python-tests.yml

@@ -70,6 +70,9 @@ jobs:
           - os: ubuntu-latest
             python-version: '3.13'
             toxenv: py313-test-pytestdev-numpydev
+          - os: ubuntu-latest
+            python-version: '3.13'
+            toxenv: py313-test-pytest83-pytestasyncio
 
     steps:
     - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2

pytest_doctestplus/plugin.py

@@ -342,6 +342,7 @@ def collect(self):
                             test.name, self, runner, test)
 
     class DocTestTextfilePlus(pytest.Module):
+        obj = None
 
         def collect(self):
             if PYTEST_GE_7_0:

tests/test_doctestplus.py

@@ -11,6 +11,13 @@
 import doctest
 from pytest_doctestplus.output_checker import OutputChecker, FLOAT_CMP
 
+try:
+    import pytest_asyncio  # noqa: F401
+    has_pytest_asyncio = True
+except ImportError:
+    has_pytest_asyncio = False
+
+
 
 pytest_plugins = ['pytester']
 
@@ -1123,6 +1130,9 @@ class MyClass:
     assert ("something()\nUNEXPECTED EXCEPTION: NameError" in report.longreprtext) is cont_on_fail
 
 
+@pytest.mark.xfail(
+        has_pytest_asyncio,
+        reason='pytest_asyncio monkey-patches .collect()')
 def test_main(testdir):
     pkg = testdir.mkdir('pkg')
     code = dedent(

tox.ini

@@ -32,6 +32,7 @@ deps =
     pytest83: pytest==8.3.*
     pytestdev: git+https://github.com/pytest-dev/pytest#egg=pytest
     numpydev: numpy>=0.0.dev0
+    pytestasyncio: pytest-asyncio
 
 extras =
     test