BUG: implement .sort_index(...inplace=True) for #11402
closes #11402

Adds inplace sort_index() for Series.

pls also add a whatsnew note for 0.17.1 (put in API changes even though this is a bug-fix, as it technically is an api change)
 Updated and running green.
 merged via https://github.com/pydata/pandas/commit/1135ce34fc2fb995e239a7d36aee1b687ef69dd9

thanks!
