Update CI configuration

[khaeru]

• Run on main, rather than master branch events.
• Add a Windows build (to catch e.g. #48).
• Add a nightly run of the pytest job—partly addresses #44, #54.
• Cancel previous workflow runs if not completed.

[codecov[bot]]

Codecov Report

Merging #73 (59e3ee4) into main (8d44cbf) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
+ Coverage   72.91%   72.93%   +0.01%     
==========================================
  Files          32       32              
  Lines        1370     1371       +1     
==========================================
+ Hits          999     1000       +1     
  Misses        371      371              

Impacted Files	Coverage Δ
item/model/dimensions.py	75.60% <100.00%> (ø)
item/remote/sdmx.py	100.00% <100.00%> (ø)

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