BUG: preserve dtypes in interpolate
check in interp_with_fill too
ENH: Add sym_diff for index

update release and docs
Merge pull request #6378 from TomAugspurger/interpolate-ignore-good3

BUG: interpolate should preserve dtypes
Merge pull request #6016 from TomAugspurger/symmetric-difference

ENH: Add sym_diff for index
API/CLN: add in common operations to Series/Index, refactored as a OpsMixin
         (GH4551, GH4056, GH5519)