FIX: Enable fixed width strings to be read from Stata 13 (117) files
Fixes a bug which prevented files containing fixed width string data from being read.  Stata 13 files also allow variable length strings, which are not supported in the current version, and an explicit exception regarding this type is now given.  Added tests which cover these cases, and Stata 13 format files. 

fixes #7360

I think this fix is ready - it is a bit ugly due to some existing PY3 compatibility code that probably has string encoding in the wrong place, but it is working.
 thanks!
