/usr/lib/ruby/2.0.0/tkextlib/setup.rb is in ruby2.0-tcltk 2.0.0.484-1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# setup.rb -- setup script before using Tk extension libraries
#
# If you need some setup operations for Tk extensions (for example,
# modify the dynamic library path) required, please write the setup
# operations in this file. This file is required at the last of
# "require 'tk'".
#
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