Python 2.6 compatibility for "{}".format(...)

[James Mills]

See below:

#!bash
prologic@daisy
Mon Apr 20 15:24:56 
~
$ python2.6
Python 2.6.9 (unknown, Aug 14 2014, 19:44:18) 
[GCC 4.8.3] on linux3
Type "help", "copyright", "credits" or "license" for more information.
>>> "{}".format("foo")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: zero length field name in format
>>> 

prologic@daisy
Mon Apr 20 15:25:10 
~
$ python2.7
Python 2.7.9 (default, Mar 19 2015, 22:32:11) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> "{}".format("foo")
'foo'
>>> 

PR coming :)