chore: update CI actions and add Dependabot

[ariostas]

The version of the actions that were used in the CI were very old and needed to be updated. I changed them all to their latest version, and I added a dependabot.yml file to keep them up to date.

Also, removed Python 3.7 from being tested since it seems to be causing some trouble (see e.g. #141). I added testing of Python 3.11 and 3.12. It would probably make sense to officially drop support for Python 3.7 soon since it has reached its end of life.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@9ed1610). Click here to learn what that means.

❗ Current head cfa2d55 differs from pull request most recent head 97ecd70. Consider uploading reports for the commit 97ecd70 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #142   +/-   ##
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  Coverage        ?   69.78%           
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  Files           ?        3           
  Lines           ?      139           
  Branches        ?        0           
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  Hits            ?       97           
  Misses          ?       42           
  Partials        ?        0           

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[jpivarski]

I approve, especially for the point of dropping old Pythons and adding new ones.

(The repo probably only needs to be tested on the minimum and maximum Python version. It's pure Python and doesn't have much functionality—mostly, it's a collection of raw files.)

[ariostas]

The repo probably only needs to be tested on the minimum and maximum Python version

Yeah, that's true. I'll change it.