Revert "No need to skip black under Python 3.7 any more"

It turns out the black tests still fail in Travis against that version.

This reverts commit d18687a30a.

tests/test_black.py

@@ -7,6 +7,10 @@ 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(