Use modern `str.format()` instead of old-style `%` string formatting

**Source:**
http://docs.python.org/2.7/library/stdtypes.html?highlight=.format#str.format

    str.format() is the new standard in Python 3, and should be preferred to the
    `%` formatting described in String Formatting Operations in new code.

    New in version 2.6.


Also makes source easier to read. ☺