Fix a bug in unicode encoding of reprs.

The previous implementation could lead to double-decoding:
    >>> from parsimonious.utils import Token
    >>> repr(Token('asdf'))
    '<Token "asdf">'
    >>> repr(Token(u'💣'))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "parsimonious/utils.py", line 19, in __repr__
        return self.__str__().encode('utf-8')
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xf0 in position 8: ordinal not in range(128)

parsimonious/utils.py

@@ -1,22 +1,15 @@
 """General tools which don't depend on other parts of Parsimonious"""
 
 import ast
-from sys import version_info
 
 from six import python_2_unicode_compatible
 
 
 class StrAndRepr(object):
-    """Mix-in to add a ``__str__`` and ``__repr__`` which return the
-    UTF-8-encoded value of ``__unicode__``"""
-
-    if version_info >= (3,):
-        # Don't return the "bytes" type from Python 3's __str__:
-        def __repr__(self):
-            return self.__str__()
-    else:
-        def __repr__(self):
-            return self.__str__().encode('utf-8')
+    """Mix-in to which gives the class the same __repr__ and __str__."""
+
+    def __repr__(self):
+        return self.__str__()
 
 
 def evaluate_string(string):