Publish package automatically (FLAPI-2004)

[sgerrand]

💁 These changes add a new workflow to publish the package to PyPI whenever a new tag is pushed to this repository.

Validation criteria

1. Add publishing workflow, using pushes, pointing to https://test.pypi.org
2. Validate that it works!
3. Update the workflow to add a constraint for git tags, add live credentials, remove https://test.pypi.org
4. Validate that it works!

[jesse-c]

Looks reasonable so far.

[sgerrand]

🏁 Successfully published a spurious version to the test version of PyPI in https://github.com/duffelhq/duffel-api-python/runs/4596393584?check_suite_focus=true.

[sgerrand]

I'll make the publishing workflow require the test workflow to pass first.

[sgerrand]

Reposting the commit message here for visibility!

The criteria for automating the steps required for publishing this
package to PyPI are:

1. The test suite should pass first
2. The publish job should only run when a tag is pushed to ensure that versions have matching tags in version control
3. Packages should only be published from duffelhq/duffel-api-python to prevent issues for contributors who may fork this repository

I'd thought that I could use separate workflows to achieve this but after spending time in GitHub's documentation for Actions I think that it's immeasurably simpler to do this all in a single workflow.

[sgerrand]

Now to test a release!

[sgerrand]

🏁 Okay, it all works!

[sgerrand]

✅ Successfully published a new version to PyPI using this new workflow job: https://github.com/duffelhq/duffel-api-python/runs/4597586080?check_suite_focus=true

[jesse-c]

LGTM

[sgerrand]

Norberto, (if you have time!) let me know what you think of this approach.

[nlopes]

Happy with the approach.