Try and group dependabot PRs

[davidorme]

This PR updates the dependabot.yml file to try and make use of the recentish PR grouping options.

I've experimentally tried to keep sphinx major version updates out of the mix.

I guess this is a bit of a "suck it and see" PR!

Fixes #591

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.18%. Comparing base (028b49d) to head (2a0a35e).
Report is 10 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #592   +/-   ##
========================================
  Coverage    95.18%   95.18%           
========================================
  Files           74       74           
  Lines         4380     4380           
========================================
  Hits          4169     4169           
  Misses         211      211           

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[jacobcook1995]

LGTM, I guess we just need to wait and see if this actually takes effect

[davidorme]

Really wish I'd merged this before dependabot kicked off this morning.