Avoid race conditions in tests using the demo_pkg_inline fixture

By using FileLock (already used as a dependency),
we ensure only one test uses the demo_pkg at a time.

This avoids race conditions happening with `pytest -n=auto`.

Fixes #2985

docs/changelog/2985.bugfix.rst

@@ -0,0 +1 @@
+Avoid race conditions in tests using the ``demo_pkg_inline`` fixture.

tests/conftest.py

@@ -3,14 +3,15 @@
 import os
 import sys
 from pathlib import Path
-from typing import Callable, Iterator, Sequence
+from typing import Callable, Generator, Iterator, Sequence
 from unittest.mock import patch
 from uuid import uuid4
 
 import pytest
 from _pytest.monkeypatch import MonkeyPatch  # cannot import from tox.pytest yet
 from _pytest.tmpdir import TempPathFactory
 from distlib.scripts import ScriptMaker
+from filelock import FileLock
 from pytest_mock import MockerFixture
 from virtualenv import cli_run
 
@@ -77,8 +78,10 @@ def demo_pkg_setuptools() -> Path:
 
 
 @pytest.fixture(scope="session")
-def demo_pkg_inline() -> Path:
-    return HERE / "demo_pkg_inline"
+def demo_pkg_inline() -> Generator[Path, None, None]:
+    demo_path = HERE / "demo_pkg_inline"
+    with FileLock(str(demo_path) + ".lock"):
+        yield demo_path
 
 
 @pytest.fixture()