Fixed parsing ellipsis as an expression in Python 3 (PY-8257)

In Python 3 ellipsis can appear anywhere, not only in sequence slicings. The parser
now always passes ellipsis, it's incorrect use is detected at the unsupported features
annotator or compatibility inspection levels.

python/testData/psi/EllipsisAsStatement.py

@@ -1,5 +0,0 @@
-def a():
-  ...     # <- highlighted as "Statement seems to have no effect"
-  pass
-
-a()

python/testData/psi/EllipsisAsStatement.txt

@@ -1,33 +0,0 @@
-PyFile:EllipsisAsStatement.py
-  PyFunction('a')
-    PsiElement(Py:DEF_KEYWORD)('def')
-    PsiWhiteSpace(' ')
-    PsiElement(Py:IDENTIFIER)('a')
-    PyParameterList
-      PsiElement(Py:LPAR)('(')
-      PsiElement(Py:RPAR)(')')
-    PsiElement(Py:COLON)(':')
-    PsiWhiteSpace('\n  ')
-    PyStatementList
-      PsiElement(Py:DOT)('.')
-      PsiErrorElement:Statement expected, found Py:DOT
-        <empty list>
-      PsiElement(Py:DOT)('.')
-      PsiErrorElement:Statement expected, found Py:DOT
-        <empty list>
-      PsiElement(Py:DOT)('.')
-      PsiErrorElement:Statement expected, found Py:DOT
-        <empty list>
-      PsiWhiteSpace('     ')
-      PsiComment(Py:END_OF_LINE_COMMENT)('# <- highlighted as "Statement seems to have no effect"')
-      PsiWhiteSpace('\n  ')
-      PyPassStatement
-        PsiElement(Py:PASS_KEYWORD)('pass')
-  PsiWhiteSpace('\n\n')
-  PyExpressionStatement
-    PyCallExpression: a
-      PyReferenceExpression: a
-        PsiElement(Py:IDENTIFIER)('a')
-      PyArgumentList
-        PsiElement(Py:LPAR)('(')
-        PsiElement(Py:RPAR)(')')

python/testData/psi/EllipsisPython3.py

@@ -0,0 +1,5 @@
+...
+x = ...
+[1, 2, ...]
+def f():
+    ...

python/testData/psi/EllipsisPython3.txt

@@ -0,0 +1,50 @@
+PyFile:EllipsisPython3.py
+  PyExpressionStatement
+    PyNoneLiteralExpression
+      PsiElement(Py:DOT)('.')
+      PsiElement(Py:DOT)('.')
+      PsiElement(Py:DOT)('.')
+  PsiWhiteSpace('\n')
+  PyAssignmentStatement
+    PyTargetExpression: x
+      PsiElement(Py:IDENTIFIER)('x')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:EQ)('=')
+    PsiWhiteSpace(' ')
+    PyNoneLiteralExpression
+      PsiElement(Py:DOT)('.')
+      PsiElement(Py:DOT)('.')
+      PsiElement(Py:DOT)('.')
+  PsiWhiteSpace('\n')
+  PyExpressionStatement
+    PyListLiteralExpression
+      PsiElement(Py:LBRACKET)('[')
+      PyNumericLiteralExpression
+        PsiElement(Py:INTEGER_LITERAL)('1')
+      PsiElement(Py:COMMA)(',')
+      PsiWhiteSpace(' ')
+      PyNumericLiteralExpression
+        PsiElement(Py:INTEGER_LITERAL)('2')
+      PsiElement(Py:COMMA)(',')
+      PsiWhiteSpace(' ')
+      PyNoneLiteralExpression
+        PsiElement(Py:DOT)('.')
+        PsiElement(Py:DOT)('.')
+        PsiElement(Py:DOT)('.')
+      PsiElement(Py:RBRACKET)(']')
+  PsiWhiteSpace('\n')
+  PyFunction('f')
+    PsiElement(Py:DEF_KEYWORD)('def')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:IDENTIFIER)('f')
+    PyParameterList
+      PsiElement(Py:LPAR)('(')
+      PsiElement(Py:RPAR)(')')
+    PsiElement(Py:COLON)(':')
+    PsiWhiteSpace('\n    ')
+    PyStatementList
+      PyExpressionStatement
+        PyNoneLiteralExpression
+          PsiElement(Py:DOT)('.')
+          PsiElement(Py:DOT)('.')
+          PsiElement(Py:DOT)('.')