API: default value for read_excel sheet (GH6573)
ENH/TST: Support for non-ascii encodings in DataFrame.to_excel (GH3710)
Merge branch 'excel_default' of https://github.com/clarkfitzg/pandas into clarkfitzg-excel_default
BUG/API: allow TimeGrouper with other columns in a groupby (GH3794)
CLN/API: replace groupby.CustomGrouper with Grouper
         rename internally Grouper to _Grouper to avoid conflict
         TimeGrouper to now inherit from Grouper