do not check identifiers for decodability

Cython/Compiler/ExprNodes.py

@@ -903,7 +903,7 @@ class StringNode(PyConstNode):
     # A Python str object, i.e. a byte string in Python 2.x and a
     # unicode string in Python 3.x
     #
-    # value          BytesLiteral
+    # value          BytesLiteral or EncodedString
     # is_identifier  boolean
 
     type = Builtin.str_type
@@ -919,6 +919,7 @@ class StringNode(PyConstNode):
             self.check_for_coercion_error(dst_type, fail=True)
 
         # this will be a unicode string in Py3, so make sure we can decode it
+        if not self.is_identifier:
             encoding = self.value.encoding or 'UTF-8'
             try:
                 self.value.decode(encoding)