This brings in support for coarsen.count as well as passing skipna down to the
other reduction functions.

* upstream/master: (22 commits)
  Added fill_value for unstack (pydata#3541)
  Add DatasetGroupBy.quantile (pydata#3527)
  ensure rename does not change index type (pydata#3532)
  Leave empty slot when not using accessors
  interpolate_na: Add max_gap support. (pydata#3302)
  units & deprecation merge (pydata#3530)
  Fix set_index when an existing dimension becomes a level (pydata#3520)
  add Variable._replace (pydata#3528)
  Tests for module-level functions with units (pydata#3493)
  Harmonize `FillValue` and `missing_value` during encoding and decoding steps (pydata#3502)
  FUNDING.yml (pydata#3523)
  Allow appending datetime & boolean variables to zarr stores (pydata#3504)
  warn if dim is passed to rolling operations. (pydata#3513)
  Deprecate allow_lazy (pydata#3435)
  Recursive tokenization (pydata#3515)
  format indexing.rst code with black (pydata#3511)
  add missing pint integration tests (pydata#3508)
  DOC: update bottleneck repo url (pydata#3507)
  add drop_sel, drop_vars, map to api.rst (pydata#3506)
  remove syntax warning (pydata#3505)
  ...

…sen-reduce

* 'coarsen-reduce' of github.com:dcherian/xarray:

Co-Authored-By: keewis <keewis@users.noreply.github.com>

* upstream/master: (22 commits)
  Resolve the version issues on RTD (pydata#3589)
  Add bottleneck & rasterio git tip to upstream-dev CI (pydata#3585)
  update whats-new.rst (pydata#3581)
  Examples for quantile (pydata#3576)
  add cftime intersphinx entries (pydata#3577)
  Add pyXpcm to Related Projects doc page (pydata#3578)
  Reimplement quantile with apply_ufunc (pydata#3559)
  add environment file for binderized examples (pydata#3568)
  Add drop to api.rst under pending deprecations (pydata#3561)
  replace duplicate method _from_vars_and_coord_names (pydata#3565)
  propagate indexes in to_dataset, from_dataset (pydata#3519)
  Switch examples to notebooks + scipy19 docs improvements (pydata#3557)
  fix whats-new.rst (pydata#3554)
  Tweaks to release instructions (pydata#3555)
  Clarify conda environments for new contributors (pydata#3551)
  Revert to dev version
  0.14.1 whatsnew (pydata#3547)
  sparse option to reindex and unstack (pydata#3542)
  Silence sphinx warnings (pydata#3516)
  Numpy 1.18 support (pydata#3537)
  ...

…sen-reduce

* 'coarsen-reduce' of github.com:dcherian/xarray:
  Update doc/whats-new.rst