ci: add a tox.ini and pyproject.toml (black) files

pyproject.toml

+[tool.black]
+target-version = ['py37']
+line-length = 100
+exclude = '''(
+    \(
+        dist
+        | sphinx
+        | \.tox
+        | \.hg
+        | \.eggs
+        | \.pytest_cache
+    \)
+)'''

tox.ini

+[tox]
+envlist = flake8,black
+
+[testenv:flake8]
+basepython = python3
+skip_install = true
+deps =
+  flake8
+commands = flake8
+
+[testenv:black]
+basepython = python3
+skip_install = true
+deps =
+  black
+commands = black --check .
+
+[flake8]
+exclude = doc/*,.tox/*
+max-line-length = 100
+extend-ignore = E203, E731, E231