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/inspections/PyCompatibilityInspection/ellipsisAsStatementPy2.py

@@ -0,0 +1,2 @@
+def foo():
+    <warning descr="Python versions < 3.0 do not support '...' outside of sequence slicings.">...</warning>

python/testData/inspections/PyCompatibilityInspection/ellipsisInSubscriptionPy2.py

@@ -0,0 +1,5 @@
+import numpy
+
+x = numpy.zeros((3, 4, 5))
+y = x[..., 0]  # pass
+y = x[..., 0, :]  # pass