bpo-40847: Consider a line with only a LINECONT a blank line (GH-20769)

A line with only a line continuation character should be considered
a blank line at tokenizer level so that only a single NEWLINE token
gets emitted. The old parser was working around the issue, but the
new parser threw a `SyntaxError` for valid input. For example,
an empty line following a line continuation character was interpreted
as a `SyntaxError`.

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>
(cherry picked from commit 896f4cf63f)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

Lib/test/test_syntax.py

@@ -854,6 +854,20 @@ class SyntaxTestCase(unittest.TestCase):
                           "iterable argument unpacking follows "
                           "keyword argument unpacking")
 
+    def test_empty_line_after_linecont(self):
+        # See issue-40847
+        s = r"""\
+pass
+        \
+
+pass
+"""
+        try:
+            compile(s, '<string>', 'exec')
+        except SyntaxError:
+            self.fail("Empty line after a line continuation character is valid.")
+
+
 def test_main():
     support.run_unittest(SyntaxTestCase)
     from test import test_syntax