Wagtail 64 upgrade

[nickmoreton]

This pull request includes several updates to dependencies and configurations, as well as improvements to the development and testing setup.

Dependency updates:

• Updated black to version 25.1.0, isort to version 6.0.1, and flake8 to version 7.1.2.

Development and testing setup:

• Added a new development_extras list with black, flake8, and isort dependencies, and modified extras_require to include development.
• Added support for Python 3.13 and Django 5.1, and updated Wagtail versions to include 6.3 and 6.4. Removed support for Python 3.8, Django 3.2, and Wagtail 6.0.

Documentation updates:

• Updated the pip install command to include the development extras.
• Changelog updated

Actions updates:

• Remove python 3.8 and add python 3.13

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.68%. Comparing base (bce562a) to head (0d5c3da).
Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   73.33%   73.68%   +0.35%     
==========================================
  Files           5        5              
  Lines          75       76       +1     
==========================================
+ Hits           55       56       +1     
  Misses         20       20              

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[ianmeigh]

Suggestion: The setup config file needs updating to match the new minimum requirements.

[ianmeigh]

Looks good @nickmoreton, I've left one suggestion.