BUG: read_fwf: incorrect error message with no colspecs or widths
Merge pull request #4774 from jtratner/fix_fwf_error_msg

BUG: read_fwf: incorrect error message with no colspecs or widths
ENH: Add 'add_metaclass' class decorator.

Taken from a new addition to the six library (again, license is in the
LICENSE directory).
CLN: Make ExcelWriter more pluggable

Make ExcelWriter an ABC and add ExcelWriter config to core/config_init