test: ignore Python 3.15 fork-with-threads DeprecationWarning in test_multiprocessing (#4143)

Python 3.15 makes os.fork() emit a DeprecationWarning when the process
has multiple threads (python/cpython#84559). zarr's sync event-loop
thread is always running by the time test_multiprocessing forks, and the
suite runs with filterwarnings = ["error"], so the fork parametrizations
fail on 3.15. Fork-safety despite those threads is exactly what this
test pins down (zarr registers an os.register_at_fork handler), so keep
the coverage and ignore the warning with a targeted per-param filter.

Fixes #4086

Assisted-by: ClaudeCode:claude-fable-5

Author: d-v-b

changes/4086.misc.md

@@ -0,0 +1 @@
+Fix `test_multiprocessing[fork]` failing on Python 3.15, where `os.fork()` in a multi-threaded process emits a `DeprecationWarning` that the test suite promotes to an error.

tests/test_array.py

@@ -1903,9 +1903,18 @@ def _index_array(arr: AnyArray, index: Any) -> Any:
     [
         pytest.param(
             "fork",
-            marks=pytest.mark.skipif(
-                sys.platform in ("win32", "darwin"), reason="fork not supported on Windows or OSX"
-            ),
+            marks=[
+                pytest.mark.skipif(
+                    sys.platform in ("win32", "darwin"),
+                    reason="fork not supported on Windows or OSX",
+                ),
+                # Python 3.15 deprecates fork() in multi-threaded processes, and zarr's
+                # sync event-loop thread is always running here. Fork-safety despite
+                # those threads is exactly what this test pins down, so keep running it.
+                pytest.mark.filterwarnings(
+                    r"ignore:This process \(pid=\d+\) is multi-threaded, use of fork\(\):DeprecationWarning"
+                ),
+            ],
         ),
         "spawn",
         pytest.param(