Unit test enforcing black formatting

Only runs for Python 3.6 at the moment.

See https://github.com/python/black/issues/425

tests/test_black.py

@@ -0,0 +1,19 @@
+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(
+        black.main, [str(code_root / "tests"), str(code_root / "datasette"), "--check"]
+    )
+    assert result.exit_code == 0, result.output