Apply black to everything, enforce via unit tests (#449)

I've run the black code formatting tool against everything:

    black tests datasette setup.py

I also added a new unit test, in tests/test_black.py, which will fail if the code does not
conform to black's exacting standards.

This unit test only runs on Python 3.6 or higher, because black itself doesn't run on 3.5.

tests/conftest.py

@@ -8,3 +8,10 @@ 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"]
+    if test_black:
+        items.insert(0, items.pop(items.index(test_black[0])))