API: change default return_type of Series.str.split from series to frame

[billletson]

The result_type argument of Series.str.split has a default value of 'series' for compatibility purposes. This should be deprecated in favor of the default 'frame'.

See issue #8428 , PR #8663 for introduction of the argument and discussion of default value.

[wesm]

The argument is now a flag expand and has a default value of false