PY-16968 Consider variadic keyword arguments mapped to keyword-only parameters

We already to it for regular (positional/keyword) parameters even
though (**kwargs) is not strictly compatible with (arg1, arg2, ...).
Now we do it for (*, kwarg1, kwarg2, ...). We don't know much about
**kwargs so it's better not to show any warnings here.

python/src/com/jetbrains/python/psi/impl/PyCallExpressionHelper.java

@@ -921,6 +921,7 @@ public class PyCallExpressionHelper {
           }
           else {
             parametersMappedToVariadicKeywordArguments.add(parameter);
+            mappedVariadicArgumentsToParameters = true;
           }
         }
         else {

python/testData/inspections/PyArgumentListInspection/kwargsAgainstKeywordOnly.py

@@ -0,0 +1,10 @@
+def f1(arg):
+    return 2
+
+def f2(*, arg):
+    return 2
+
+
+def test(**kwargs):
+    f1(**kwargs)
+    f2(**kwargs)

python/testSrc/com/jetbrains/python/inspections/PyArgumentListInspectionTest.java

@@ -322,6 +322,11 @@ public class PyArgumentListInspectionTest extends PyTestCase {
     runWithLanguageLevel(LanguageLevel.PYTHON35, this::doTest);
   }
 
+  // PY-16968
+  public void testKwargsAgainstKeywordOnly() {
+    runWithLanguageLevel(LanguageLevel.PYTHON36, this::doTest);
+  }
+
   private void doMultiFileTest() {
     final String folderPath = "inspections/PyArgumentListInspection/" + getTestName(false) + "/";