CLN: Remove py3 next method from FixedWidthReader

It's unnecessary - handle wraps this instead.
Implementation of Stata 13 (format 117) support
Updated docs
Fixed docstring of read_stata
Removed unused imports
Fixed some bugs in Stata format 117 parsing
Added unit testing for Stata 13 format
Merge pull request #4923 from jtratner/simplify-next-in-fwf

CLN: Remove py3 next method from FixedWidthReader
Merge pull request #4662 from PKEuS/master

Added Stata 13 support
DOC: v0.13.0 for stata enhancement