Encoding output if it's Unicode only for python2

src/flake8/formatting/base.py

@@ -1,6 +1,7 @@
 """The base class and interface for all formatting plugins."""
 from __future__ import print_function
 
+import sys
 
 class BaseFormatter(object):
     """Class defining the formatter interface.
@@ -166,7 +167,7 @@ class BaseFormatter(object):
 
     def _write(self, output):
         """Handle logic of whether to use an output file or print()."""
-        if not isinstance(output, str):
+        if sys.version_info < (3, 0) and isinstance(output, unicode):
             output = output.encode('UTF-8')
         if self.output_fd is not None:
             self.output_fd.write(output + self.newline)