Merge branch 'generate-reductions-class' into groupby-aggs-using-numpy-groupies

* generate-reductions-class: (31 commits)
  polish
  update _reductions
  Fixes
  Apply suggestions from code review
  Fix path
  manual tweaks to make ci happy
  Update _reductions.py
  Write to file using open() instead.
  force keyword args after dim
  Annotate some reduction tests.
  Update xarray/util/generate_reductions.py
  Update xarray/util/generate_reductions.py
  add doctests
  more reduce
  another reduce
  one more reduce
  more reduce edits
  make reduce args consistent
  updates
  [pre-commit.ci] auto fixes from pre-commit.com hooks
  ...

Author: dcherian