Don't run black test on Python 3.7

It breaks on that version - see psf/black#425

But that's OK, we only need it to run onder one version anyway.

tests/test_black.py

@@ -1,10 +1,16 @@
 import black
 from click.testing import CliRunner
 from pathlib import Path
+import pytest
+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(