DEPR: Series.put, Series.real, Series.imag, Index.dtype_str
- [x] xref #https://github.com/pandas-dev/pandas/issues/18262
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Removes:
`Series.put`
`Index.dtype_str`
`Series.real`
`Series.imag`