Switch master to main in CI and docs

[felixcremer]

This closes #555 @lazarusA

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@71a1278). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #557   +/-   ##
=======================================
  Coverage        ?   69.80%           
=======================================
  Files           ?       14           
  Lines           ?     2113           
  Branches        ?        0           
=======================================
  Hits            ?     1475           
  Misses          ?      638           
  Partials        ?        0           

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[lazarusA]

all good. Only you went one master too many!

this one is not: https://github.com/pydata/xarray-data/raw/main/rasm.nc

but https://github.com/pydata/xarray-data/raw/master/rasm.nc with master :D