Ignore editable installation modules (#10231)

Fixes #10230

changelog/10230.bugfix.rst

@@ -0,0 +1 @@
+Ignore ``.py`` files created by ``pyproject.toml``-based editable builds introduced in `pip 21.3 <https://pip.pypa.io/en/stable/news/#v21-3>`__.

src/_pytest/config/__init__.py

@@ -836,7 +836,8 @@ def _iter_rewritable_modules(package_files: Iterable[str]) -> Iterator[str]:
         if is_simple_module:
             module_name, _ = os.path.splitext(fn)
             # we ignore "setup.py" at the root of the distribution
-            if module_name != "setup":
+            # as well as editable installation finder modules made by setuptools
+            if module_name != "setup" and not module_name.startswith("__editable__"):
                 seen_some = True
                 yield module_name
         elif is_package:

testing/test_config.py

@@ -842,6 +842,9 @@ def test_confcutdir_check_isdir(self, pytester: Pytester) -> None:
             (["src/bar/__init__.py"], ["bar"]),
             (["src/bar/__init__.py", "setup.py"], ["bar"]),
             (["source/python/bar/__init__.py", "setup.py"], ["bar"]),
+            # editable installation finder modules
+            (["__editable___xyz_finder.py"], []),
+            (["bar/__init__.py", "__editable___xyz_finder.py"], ["bar"]),
         ],
     )
     def test_iter_rewritable_modules(self, names, expected) -> None: