Revision on assignment errors (#97)

* Revision on assignment expression errors

 * added rule for __debug__ (should be a keyword)
 * reviewed error messages
 * added new failing samples

* Adjustment upon Dave's review

 * rewind several changes in assignment errors
 * patched is_definition: command not found for assignment expressions
 * patched Python 2 inconsistent error messages in test_python_errors.py: command not found

test/failing_examples.py

@@ -347,4 +347,14 @@ if sys.version_info[:2] >= (3, 8):
         # Not in that issue
         '(await a := x)',
         '((await a) := x)',
+        # new discoveries
+        '((a, b) := (1, 2))',
+        '([a, b] := [1, 2])',
+        '({a, b} := {1, 2})',
+        '({a: b} := {1: 2})',
+        '(a + b := 1)',
+        '(True := 1)',
+        '(False := 1)',
+        '(None := 1)',
+        '(__debug__ := 1)',
     ]