Add auto-formatting via black

Introduce black, a Python code formatter. Also a Makefile that
includes two common targets:

* fmt (auto-format the code)
* lint (lint the code and return non-zero if out of spec)

We can re-use the latter in CI.

By introducing a code formatter, we can cut down on stylistic
variations across a now-large codebase. We can also avoid most style
discussions in PR if we just have this thing enforce it for us.

requirements.txt

@@ -9,9 +9,10 @@ inflect
 autobahn >= 17.9.3
 model_mommy
 
-# testing
+# testing and development
 mock >= 1.0.1
 anything
+black
 
 # windows-specific
 pypiwin32;platform_system=="Windows"