* upstream/master: (54 commits)
  Limit repr of arrays containing long strings (pydata#3900)
  expose a few zarr backend functions as semi-public api (pydata#3897)
  Use drawstyle instead of linestyle in plot.step. (pydata#3274)
  Implementation of polyfit and polyval (pydata#3733)
  misplaced quote in whatsnew (pydata#3889)
  Rename ordered_dict_intersection -> compat_dict_intersection (pydata#3887)
  Control attrs of result in `merge()`, `concat()`, `combine_by_coords()` and `combine_nested()` (pydata#3877)
  xfail test_uamiv_format_write (pydata#3885)
  Use `fixes` in PR template (pydata#3886)
  Tweaks to "how_to_release" (pydata#3882)
  whatsnew section for 0.16.0
  Release v0.15.1
  whatsnew for 0.15.1 (pydata#3879)
  update panel documentation (pydata#3880)
  reword the whats-new entry for unit support (pydata#3878)
  Raise error when assigning to IndexVariable.values & IndexVariable.data (pydata#3862)
  Re-enable tests xfailed in pydata#3808 and fix new CFTimeIndex failures due to upstream changes (pydata#3874)
  add spacing in the versions section of the issue report (pydata#3876)
  map_blocks: allow user function to add new unindexed dimension. (pydata#3817)
  Delete associated indexes when deleting coordinate variables. (pydata#3840)
  ...

* upstream/master: (75 commits)
  Implement idxmax and idxmin functions (pydata#3871)
  Update pre-commit-config.yaml (pydata#3911)
  Revert "Use `fixes` in PR template (pydata#3886)" (pydata#3912)
  update the docstring of diff (pydata#3909)
  Un-xfail test_dayofyear_after_cftime_range (pydata#3907)
  Limit repr of arrays containing long strings (pydata#3900)
  expose a few zarr backend functions as semi-public api (pydata#3897)
  Use drawstyle instead of linestyle in plot.step. (pydata#3274)
  Implementation of polyfit and polyval (pydata#3733)
  misplaced quote in whatsnew (pydata#3889)
  Rename ordered_dict_intersection -> compat_dict_intersection (pydata#3887)
  Control attrs of result in `merge()`, `concat()`, `combine_by_coords()` and `combine_nested()` (pydata#3877)
  xfail test_uamiv_format_write (pydata#3885)
  Use `fixes` in PR template (pydata#3886)
  Tweaks to "how_to_release" (pydata#3882)
  whatsnew section for 0.16.0
  Release v0.15.1
  whatsnew for 0.15.1 (pydata#3879)
  update panel documentation (pydata#3880)
  reword the whats-new entry for unit support (pydata#3878)
  ...