0.8.0rc1

Prep 0.8 (#689)

* Fix pr ref

* draft

* Typo fix

* note removal of xlrd dep

* Remove datetime addition from release notes

* Add read_elem, write_elem to release (breaks build)

Added `read_elem`, `write_elem` to experimental and release notes.

Breaks build due to typing in docs.

* Make docs build

* A little cleanup around X=None

* Bump required version of h5py (tentative change)

* pandas 1.4.0 compat (only test suite was broken, probably don't need release)

* Note release candidate is out