textproc/py-tablib: Update to 0.13.0

While I'm here:

• Remove the dependency for math/py-pandas because it's listed as an extra dependency and thus not required for basic runtime.
• Add a note for users that want to make use of the Pandas Dataframe structure.
• Add a patch for setup.py to limit devel/py-backports.csv to Python 2.
• Pet portlint

Changelog:

https://github.com/kennethreitz/tablib/compare/v0.12.1...v0.13.0

Approved by: mentors (implicit)