Handle tags created through GitHub in publish release workflow

[nawatts]

Currently, the GitHub Actions workflow for publishing a release assumes that the version is tag is created locally and pushed to GitHub. It automatically creates a corresponding GitHub Release with automatically generated release notes.

/~https://github.com/broadinstitute/gnomad_methods/blob/00365eb7b11ae1f93ad865f1c56917b04c19006f/CONTRIBUTING.md#publishing-a-release

gnomad_methods/.github/workflows/publish.yml

Lines 50 to 57 in 00365eb

	- name: Create release notes
	uses: actions/github-script@v5
	with:
	script: |
	await github.request(`POST /repos/${{ github.repository }}/releases`, {
	tag_name: "${{ github.ref }}",
	generate_release_notes: true
	});

However, it's also possible to create a tag through GitHub by manually creating a GitHub Release.

In that case, the last step of the release workflow fails because a release already exists for the tag. This happened for the 0.6.2 release and caused confusion because, while everything ended in the desired state (version tagged, code published to PyPI, and release notes in a GitHub Release), the publish release workflow showed as failed.

This changes the publish release workflow to handled the case where a tag is created through GitHub by first checking if a GitHub Release already exists for the version tag before attempting to create one.

[ch-kr]

one small (optional) suggestion, but LGTM. thank you for adding this!!