Call black directly (not via flake8)

Now that the entire code base uses black this
is practical - the flake8 configuration gave
a convienent way to manage the gradual adoption.

However, right now my flake8-black plugin has
more overhead than it should and slows this down.
peterjc/flake8-black#26

.travis-tox.ini

@@ -102,8 +102,8 @@ deps =
     flake8-comprehensions
     flake8-bugbear
     flake8-implicit-str-concat
-    flake8-black
     flake8-quotes
+    black
     restructuredtext_lint
     doc8
     pygments
@@ -116,6 +116,8 @@ deps =
 commands =
     # PEP-8 and PEP-257 style checks:
     flake8
+    # Black style check
+    black --check .
     # Now do various checks on our RST files:
     # Calling via bash to get it to expand the wildcard for us
     bash -c \'rst-lint --level warning *.rst\'