Add pooch downloader to workaround RtD issue

[znichollscr]

Issue is here: readthedocs/readthedocs.org#11763

This may bite in this repository soon, but was causing issues for me as a downstream user here: climate-resource/input4mips_validation#84

Fix from Mathias Hauser is here, that's what I've implemented here: readthedocs/readthedocs.org#11763 (comment)

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.25%. Comparing base (a9cebee) to head (ef33569).
Report is 47 commits behind head on main.

Files with missing lines	Patch %	Lines
cf_xarray/utils.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #547      +/-   ##
==========================================
+ Coverage   85.78%   86.25%   +0.46%     
==========================================
  Files          13       15       +2     
  Lines        2364     2953     +589     
  Branches      183      256      +73     
==========================================
+ Hits         2028     2547     +519     
- Misses        303      361      +58     
- Partials       33       45      +12     

Flag	Coverage Δ
mypy	43.65% <0.00%> (+5.11%)	⬆️
unittests	93.36% <100.00%> (-0.62%)	⬇️

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