Fix isort invocation referring to gone setup.cfg file (#922).

tox.ini

@@ -14,7 +14,7 @@ passenv = COVERAGE_FILE
 commands =
     pytest {posargs}
     black --check .
-    isort --check-only --settings-path setup.cfg .
+    isort --check-only .
     flake8 borgmatic tests
     codespell
 
@@ -39,7 +39,7 @@ commands =
 [testenv:isort]
 deps = {[testenv]deps}
 commands =
-    isort --settings-path setup.cfg .
+    isort .
 
 [testenv:codespell]
 deps = {[testenv]deps}