tox: Enable testing and linting ngclient

* Use the same rules as tuf/api
* omit ngclient from coverage limits for now: theupdateframework#1309

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

tox.ini

@@ -16,7 +16,7 @@ changedir = tests
 commands =
     python --version
     python -m coverage run aggregate_tests.py
-    python -m coverage report -m --fail-under 97
+    python -m coverage report -m --fail-under 97 --omit "{toxinidir}/tuf/ngclient/*"
 
 deps =
     -r{toxinidir}/requirements-test.txt
@@ -43,13 +43,13 @@ 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
-    isort --check --diff --line-length 80 --profile black -p tuf tuf/api
-    pylint -j 0 tuf/api --rcfile=tuf/api/pylintrc
+    black --check --diff --line-length 80 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=tuf/api/pylintrc
 
     # NOTE: Contrary to what the pylint docs suggest, ignoring full paths does
     # work, unfortunately each subdirectory has to be ignored explicitly.
-    pylint -j 0 tuf --ignore=tuf/api,tuf/api/serialization
+    pylint -j 0 tuf --ignore=tuf/api,tuf/api/serialization,tuf/ngclient,tuf/ngclient/_internal
 
     mypy