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/psi-api/src/com/jetbrains/python/psi/PyNoneLiteralExpression.java

@@ -6,4 +6,5 @@ package com.jetbrains.python.psi;
  * @author yole
  */
 public interface PyNoneLiteralExpression extends PyLiteralExpression {
+  boolean isEllipsis();
 }