Add an initial black <https://black.readthedocs.io> setup in a PEP 518 pyproject.toml file.

pyproject.toml

+[tool.black]
+line-length = 79
+py36 = true
+skip-string-normalization = true
+include = '(diffoscope|tests)/.+\.pyi?$'
+exclude = "diffoscope/changes.py"