Revert "revert conftest.py"

This reverts commit 0f86c59.

Author: ieivanov

tests/conftest.py

@@ -1,6 +1,7 @@
 import csv
 import os
 import shutil
+import sys
 from pathlib import Path
 
 import fsspec
@@ -13,6 +14,19 @@
 settings.load_profile("default")
 
 
+# Make the repo root importable from `multiprocessing` spawn children so that
+# tests using ProcessPoolExecutor (e.g. test_process_single_position with
+# use_threads=False) can unpickle helpers like `tests.ngff.test_ngff_utils.
+# dummy_transform`. pytest's `--import-mode=importlib` only manipulates the
+# parent process's sys.path, not the env that spawn children inherit.
+_REPO_ROOT = Path(__file__).resolve().parent.parent
+os.environ["PYTHONPATH"] = os.pathsep.join(
+    [str(_REPO_ROOT)] + ([os.environ["PYTHONPATH"]] if os.environ.get("PYTHONPATH") else [])
+)
+if str(_REPO_ROOT) not in sys.path:
+    sys.path.insert(0, str(_REPO_ROOT))
+
+
 @pytest.fixture
 def rng():
     return np.random.default_rng(42)