Copy GHA config for releasing to PyPI from s3 storage provider

[DMRobertson]

matrix-org/synapse-s3-storage-provider#69 and matrix-org/synapse-s3-storage-provider#70 but for the generic template.

This will have the following effects:

• modules created with the template will have a CI step checking they can be built into a distribution package
• publishing a release will attempt to publish to PYPI. This requires the repository to have a PYPI_API_TOKEN secret set and will fail otherwise.

[squahtx]

Do we want to update the release instructions in the templated README too?
We could do it as part of a different PR if you prefer.

[squahtx]

Does the Python version matter here? I suppose not.

[squahtx]

nitty nit: We've got difference whitespace styles here and in ci.yml. As a template I'd prefer it if we were consistent.

[DMRobertson]

Thanks for taking a look Sean.

On reflection I'm wondering if it might make more sense to define a backend-meta repo and stick these workflows in there, so that we can use them via github's reusable workflows. (In a nutshell, I don't want us to have to edit N yaml files across N repos if we want to update our processes.)

[DMRobertson]

Thanks for taking a look Sean.

On reflection I'm wondering if it might make more sense to define a backend-meta repo and stick these workflows in there, so that we can use them via github's reusable workflows. (In a nutshell, I don't want us to have to edit N yaml files across N repos if we want to update our processes.)

This is matrix-org/backend-meta#4.