Move black configuration in pyproject.toml

We are using 4 linters: black, isort, pylint and mypy.
It's good if we use one file as a source for truth for all linter
configurations.

As a first step move black options in pyproject.toml.
I tried multiple ways to use the include option,
so we can just call black --config=pyproject.toml, but I was not
successful. Then I found this comment psf/black#861 (comment)
explaining that the path argument is mandatory.
As a result, I will move all configuration options for black inside
pyproject.toml without the actual directories that need to be linted.

Signed-off-by: Martin Vrachev <[email protected]>

docs/CONTRIBUTORS.rst

@@ -114,7 +114,7 @@ Auto-formatting can be done on the command line:
 ::
 
     $ # TODO: configure black and isort args in pyproject.toml (see #1161)
-    $ black --line-length 80 tuf/api
+    $ black <filename>
     $ isort --line-length 80 --profile black -p tuf tuf/api
 
 or via source code editor plugin

pyproject.toml

@@ -1,7 +1,13 @@
+# Build-system section
 [build-system]
 requires = ["setuptools>=40.8.0", "wheel"]
 build-backend = "setuptools.build_meta"
 
+# Black section
+# Read more here: https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html#configuration-via-a-file
+[tool.black]
+line-length=80
+
 # Pylint section
 
 # Minimal pylint configuration file for Secure Systems Lab Python Style Guide:

tox.ini

@@ -41,7 +41,7 @@ changedir = {toxinidir}
 commands =
     # Use different configs for new (tuf/api/*) and legacy code
     # TODO: configure black and isort args in pyproject.toml (see #1161)
-    black --check --diff --line-length 80 tuf/api tuf/ngclient
+    black --check --diff tuf/api tuf/ngclient
     isort --check --diff --line-length 80 --profile black -p tuf tuf/api tuf/ngclient
     pylint -j 0 tuf/api tuf/ngclient --rcfile=pyproject.toml