Move part of isort options 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.

I tried multiple ways to use the src_path option,
so we can just call isort without pointing out the target folders, but I was not
successful.
I tried running isort with "isort --settings-path=pyproject.toml"
I got the error:
"Error: arguments passed in without any paths or content."

Additionally, I saw one project with source configuration https://github.com/Pylons/pyramid/blob/master/pyproject.toml,
but they had to give explicit folders too https://github.com/Pylons/pyramid/blob/8061fce297cc7117d3e6e2b39e47512c7db2904f/tox.ini#L26
and https://github.com/Pylons/pyramid/blob/8061fce297cc7117d3e6e2b39e47512c7db2904f/tox.ini#L66

It was a similar situation with "check" and "diff".
In the documentation it's said that for both check and diff are not
supported in configuration files.
See:
- https://pycqa.github.io/isort/docs/configuration/options.html#check
- https://pycqa.github.io/isort/docs/configuration/options.html#show-diff

Additionally, in two issues it was confirmed that in integration tests
we should use --check and --diff the way we did until now.

As a result, I moved part of the configuration options for isort inside
pyproject.toml without the actual directories that need to be linted
and "check" and "diff" options.

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

docs/CONTRIBUTORS.rst

@@ -113,9 +113,8 @@ CI/CD will check that new TUF code is formatted with `black
 Auto-formatting can be done on the command line:
 ::
 
-    $ # TODO: configure black and isort args in pyproject.toml (see #1161)
     $ black tuf/api tuf/ngclient
-    $ isort --line-length 80 --profile black -p tuf tuf/api
+    $ isort tuf/api tuf/ngclient
 
 or via source code editor plugin
 [`black <https://black.readthedocs.io/en/stable/editor_integration.html>`__,

pyproject.toml

@@ -9,6 +9,12 @@ line-length=80
 check="True"
 diff="True"
 
+# Isort section
+[tool.isort]
+profile="black"
+line_length=80
+known_first_party = ["tuf"]
+
 # Pylint section
 
 # Minimal pylint configuration file for Secure Systems Lab Python Style Guide:

tox.ini

@@ -40,9 +40,8 @@ commands =
 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 tuf/api tuf/ngclient
-    isort --check --diff --line-length 80 --profile black -p tuf tuf/api tuf/ngclient
+    isort --check --diff tuf/api tuf/ngclient
     pylint -j 0 tuf/api tuf/ngclient --rcfile=pyproject.toml
 
     # NOTE: Contrary to what the pylint docs suggest, ignoring full paths does