Add configuration file for black defaults

The `black` project decided not to support `setup.cfg`, but only
`pyproject.toml` (see psf/black#65).

In order to avoid multiple configuration places for
our `black` defaults, I added this additional configuration file to
the repo.

pyproject.toml

@@ -0,0 +1,14 @@
+[tool.black]
+line-length = 99
+target-version = ['py37']
+include = '\.pyi?$'
+exclude = '''
+/(
+    \.git
+  | \.mypy_cache
+  | \.tox
+  | \.venv
+  | build
+  | dist
+)/
+'''