Ensure test_black.py executes first under pytest

Otherwise it throws an exception because it gets disrupted by the
asyncio activity in the other tests.

tests/conftest.py

@@ -8,3 +8,9 @@ def pytest_unconfigure(config):
     import sys
 
     del sys._called_from_test
+
+
+def pytest_collection_modifyitems(items):
+    # Ensure test_black.py runs first before any asyncio code kicks in
+    test_black = [fn for fn in items if fn.name == "test_black"][0]
+    items.insert(0, items.pop(items.index(test_black)))

tests/test_black.py

@@ -7,10 +7,6 @@ import sys
 code_root = Path(__file__).parent.parent
 
 
-# @pytest.mark.skipif(
-#     sys.version_info[:2] > (3, 6),
-#     reason="Breaks on 3.7 at the moment, but it only needs to run under one Python version",
-# )
 def test_black():
     runner = CliRunner()
     result = runner.invoke(