Workaround to deal with Phython bug in 2.6.0 - 2.6.4, fixes #3813

youtube-dl imports unicode_literals from __future__, which makes
the string literals used as keys for dictionaries to be
unicode strings, but that's not supported until Python 2.6.5.
This workaround labels them as binary strings using the "b" prefix,
which keeps them as old-style strings.