Typing support for custom backends (#6651)
Typing support for custom backends (#6651)

* add str to allowed engine types

* Update xarray/backends/api.py

* Update xarray/backends/api.py

* Update xarray/backends/api.py

Co-authored-by: Illviljan <ae7d6feeeeda16e2fe885966273a27a64c0c2b0a@users.noreply.github.com>
pydata/xarray,jhamman/xarray,jhamman/xarray,jhamman/xarray,markelg/xray,pydata/xarray,pydata/xarray,markelg/xray,markelg/xray