gh-1001: Improve process for updating copyright headers

[GCHQDeveloper314]

Related Issue

• Resolve Improve process for updating copyright headers #1001

[GCHQDeveloper314]

This requires Gaffer 2 alpha 3. Once that is releases and gaffer-tools upgraded to use it, this should pass tests.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (v2-alpha@5e1a304). Click here to learn what that means.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             v2-alpha    #1002   +/-   ##
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  Coverage            ?   45.15%           
  Complexity          ?      101           
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  Files               ?       25           
  Lines               ?      897           
  Branches            ?       73           
===========================================
  Hits                ?      405           
  Misses              ?      462           
  Partials            ?       30           

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[GCHQDeveloper314]

Python tests are not passing - this will be fixed in a different PR.