[UK] Add command to add GSS codes to CED areas using CSV from ONS

[davea]

Current data from ONS: https://geoportal.statistics.gov.uk/documents/ons::county-electoral-division-to-county-may-2021-lookup-for-england/about (needs converting to CSV manually, didn't really want to introduce a dependency for parsing XLSX files)

[codecov[bot]]

Codecov Report

Merging #389 (55c59f2) into master (f668519) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #389   +/-   ##
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  Coverage   71.96%   71.96%           
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  Files          39       39           
  Lines        2297     2297           
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  Hits         1653     1653           
  Misses        644      644           

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[dracos]

Note the 2.7 flake8 test broke as the script uses a python 3 only bit of code. I know we're dropping py2 in another PR (soon presumably) so perhaps we could just drop the py2 flake8 in Actions, there's still a py3 run of it.

[davea]

Oops, yes. The {} set literal syntax (TIL this was in Python 2.7) and/or f-strings I guess. Should I drop py2 on master now or leave it until #388 lands?

[dracos]

Might as well drop it now so we don't have the failing test, even if only for a (hopefully) short time :) Looks like I didn't drop six in #388 anyway so more could be done there.