src/sage/symbolic/ring.pyx (isidentifier): Remove py2-only code, use …

…of deprecated parser module

src/sage/symbolic/ring.pyx

@@ -33,7 +33,6 @@ from sage.rings.all import RR, CC, ZZ
 
 import keyword
 import operator
-import parser
 
 # Do not allow any of these keywords as identifiers for symbolic variables
 KEYWORDS = set(keyword.kwlist).union(['exec', 'print', 'None', 'True',
@@ -1380,7 +1379,7 @@ def isidentifier(x):
     Boolean. Whether the string ``x`` can be used as a variable name.
 
     This function should return ``False`` for keywords, so we can not
-    just use the ``isidentifier`` method of strings (in Python 3),
+    just use the ``isidentifier`` method of strings,
     because, for example, it returns ``True`` for "def" and for "None".
 
     EXAMPLES::
@@ -1409,15 +1408,4 @@ def isidentifier(x):
     """
     if x in KEYWORDS:
         return False
-    try:
-        if not x.isidentifier():  # py3
-            return False
-    except AttributeError:
-        pass  # py2
-
-    try:
-        code = parser.expr(x).compile()
-    except (MemoryError, OverflowError, SyntaxError,
-            SystemError, parser.ParserError):
-        return False
-    return len(code.co_names) == 1 and code.co_names[0] == x
+    return x.isidentifier()