tweak Python parser to handle incomplete code better (PY-4053)

python/testData/psi/MissingParenInCall.py

@@ -0,0 +1,5 @@
+def get_key():
+    if isinstance(self.instance,
+
+def clean():
+    pass

python/testData/psi/MissingParenInCall.txt

@@ -0,0 +1,41 @@
+PyFile:MissingParenInCall.py
+  PyFunction('get_key')
+    PsiElement(Py:DEF_KEYWORD)('def')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:IDENTIFIER)('get_key')
+    PyParameterList
+      PsiElement(Py:LPAR)('(')
+      PsiElement(Py:RPAR)(')')
+    PsiElement(Py:COLON)(':')
+    PsiWhiteSpace('\n    ')
+    PyStatementList
+      PyIfStatement
+        PyIfPartIf
+          PsiElement(Py:IF_KEYWORD)('if')
+          PsiWhiteSpace(' ')
+          PyCallExpression: isinstance
+            PyReferenceExpression: isinstance
+              PsiElement(Py:IDENTIFIER)('isinstance')
+            PyArgumentList
+              PsiElement(Py:LPAR)('(')
+              PyReferenceExpression: instance
+                PyReferenceExpression: self
+                  PsiElement(Py:IDENTIFIER)('self')
+                PsiElement(Py:DOT)('.')
+                PsiElement(Py:IDENTIFIER)('instance')
+              PsiElement(Py:COMMA)(',')
+              PsiErrorElement:expression expected
+                <empty list>
+      PsiWhiteSpace('\n\n')
+      PyFunction('clean')
+        PsiElement(Py:DEF_KEYWORD)('def')
+        PsiWhiteSpace(' ')
+        PsiElement(Py:IDENTIFIER)('clean')
+        PyParameterList
+          PsiElement(Py:LPAR)('(')
+          PsiElement(Py:RPAR)(')')
+        PsiElement(Py:COLON)(':')
+        PsiWhiteSpace('\n    ')
+        PyStatementList
+          PyPassStatement
+            PsiElement(Py:PASS_KEYWORD)('pass')