Checks for Wagtail 6.3

[nickmoreton]

This pull request includes several updates to dependencies, Python version support, and testing configurations.

The commits from: #27 which also included #26 so they can be closed now.

Python version support:

• Removed support for Python 3.8 in .github/workflows/tests.yml, tox.ini, and setup.cfg. [1] [2] [3]

Dependency updates:

• Updated black, isort, and flake8 versions in .pre-commit-config.yaml.
• Updated setuptools and pip versions in setup.cfg.
• Updated coverage version in setup.py.
• Updated python-unsplash version in setup.cfg and setup.py. [1] [2]

Django and Wagtail support:

• Added support for Django 5.1 and Wagtail 6.x in tox.ini and CHANGELOG.md. [1] [2]

Miscellaneous:

• Removed SiteMiddleware from wagtailmakeup/tests/settings.py.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.68%. Comparing base (bce562a) to head (ea71f80).
Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
+ 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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