Mark PillowLeakTestCase as isolated

Author: hugovk

Tests/helper.py

@@ -211,6 +211,7 @@ def is_pypy() -> bool:
     return sys.implementation.name == "pypy"
 
 
+@pytest.mark.isolated
 @pytest.mark.skipif(sys.platform.startswith("win32"), reason="Requires Unix or macOS")
 # Per https://stackoverflow.com/a/29007723/51685, due to JIT compilation,
 # RSS utilization is known to grow in PyPy.

pyproject.toml

@@ -209,6 +209,9 @@ num_workers = 4
 
 [tool.pytest]
 addopts = [ "-ra", "--color=auto" ]
+markers = [
+  "isolated: tests that are not compatible with pytest-xdist (deselect with '-m \"not isolated\"')",
+]
 testpaths = [
   "Tests",
 ]