Added automerge github workflow

[jacobcook1995]

Description

This PR copies the automerge workflow that @dalonsoa suggested we could use to automatically merge dependabot and pre-commit pull requests (provided that they pass the CI/QA checks).

I can't think of anyway of testing whether this works (we just have to wait for a dependabot/pre-commit PR). So do let me know if the workflow looks like it'll work.

Fixes #570

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 95.09%. Comparing base (3c53e4e) to head (d66f6cd).
⚠️ Report is 3229 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #584   +/-   ##
========================================
  Coverage    95.09%   95.09%           
========================================
  Files           74       74           
  Lines         4302     4302           
========================================
  Hits          4091     4091           
  Misses         211      211           

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[davidorme]

LGTM