a trailing comma in argument list is not an error (PY-4016)

python/testData/psi/TrailingCommaInArgList.py

@@ -0,0 +1,2 @@
+def some_function(a,):
+    pass

python/testData/psi/TrailingCommaInArgList.txt

@@ -0,0 +1,16 @@
+PyFile:TrailingCommaInArgList.py
+  PyFunction('some_function')
+    PsiElement(Py:DEF_KEYWORD)('def')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:IDENTIFIER)('some_function')
+    PyParameterList
+      PsiElement(Py:LPAR)('(')
+      PyNamedParameter('a')
+        PsiElement(Py:IDENTIFIER)('a')
+      PsiElement(Py:COMMA)(',')
+      PsiElement(Py:RPAR)(')')
+    PsiElement(Py:COLON)(':')
+    PsiWhiteSpace('\n    ')
+    PyStatementList
+      PyPassStatement
+        PsiElement(Py:PASS_KEYWORD)('pass')