* Revert "Use _unstack_once for valid dask and sparse versions (#5315)"

This reverts commit 9165c266f52830384c399f0607a4123e65bb7803.

* 0.18.2 release notes

* fix RTD [skip-ci] (#5518)

* v0.19.0 release notes

* Update doc/whats-new.rst [skip-ci]

* remove empty sections

Co-authored-by: Maximilian Roos <[email protected]>
Co-authored-by: keewis <[email protected]>