chore: Add a test workflow

[edgarrmondragon]

This is the first PR in a series to migrate CI jobs from GitLab CI to GitHub Actions.

Checks run:

• Unit tests (tests/core, tests/cookiecutters) on all supported Python versions
• External tests (tests/external). Requires setting up secrets.
• Doctests
• Lint

For unit and external tests, coverage is computed and later combined and uploaded to Codecov in the coverage job.

This PR also adds dependabot updates for Python tools used in CI (poetry, pip) and third-party GitHub actions.

Closes #680

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@050f604). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #679   +/-   ##
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  Coverage        ?   85.18%           
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  Files           ?       34           
  Lines           ?     3361           
  Branches        ?        0           
=======================================
  Hits            ?     2863           
  Misses          ?      498           
  Partials        ?        0           

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[aaronsteers]

LGTM! Thanks for this quick pass!

One failing CI test and then this is ready to go. Thanks!

I love the codecov display - thanks for that!

[edgarrmondragon]

@aaronsteers

One failing CI test and then this is ready to go. Thanks!

Yup, that's the external tests that need some credentials.

[aaronsteers]

Yup, that's the external tests that need some credentials.

Do I need to take some action for that?

[edgarrmondragon]

Do I need to take some action for that?

@aaronsteers yup, any secrets used in the GitLab CI job should be copied to https://github.com/meltano/sdk/settings/secrets/actions

[aaronsteers]

Do I need to take some action for that?

@aaronsteers yup, any secrets used in the GitLab CI job should be copied to https://github.com/meltano/sdk/settings/secrets/actions

Secrets are now created as organization secrets, shared with this repo and present at the link above.

I've temporarily removed the external item from the matrix so this can pass.

@edgarrmondragon - can we merge for now and handle the follow-up in #683 ?

[edgarrmondragon]

@aaronsteers

can we merge for now and handle the follow-up in #683 ?

Sure 👍