Fixed an error with unicode literals in template extension setup.py

When a new extension is created using ./contrib/tools/generate_extension.py and
running 'python setup.py develop' an error "setup command: package_data must be
a dictionary mapping package names to lists of wildcard patterns" occurs. The
problem is caused by the setup.py file importing __future__ unicode literals
because Python2 setuptools expects a "str" for parsing package_data.

Testing Done:
Created a new extension and ran 'python setup.py develop' with no errors.

Reviewed at https://reviews.reviewboard.org/r/6704/
Do a different fix for setuptools' package_data.

I meant to amend the previous commit, but I missed it. This is the intended
fix. Trivial change.