tox: Run pylint in parallel

pylint on the legacy code is by far the slowest part of linting (to
the extent that parallelizing the tox env itself doesn't really help):
pylint can fortunately parallelize itself.

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: Jussi Kukkonen

tox.ini

@@ -45,11 +45,11 @@ commands =
     # 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 tuf/api --rcfile=tuf/api/pylintrc
+    pylint -j 0 tuf/api --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 tuf --ignore=tuf/api,tuf/api/serialization
+    pylint -j 0 tuf --ignore=tuf/api,tuf/api/serialization
 
     mypy