.github/workflows/main.yml: Add portability/integration testing with Sage

[mkoeppe]

We use the reusable workflows from Sage to test portability and integration with Sage.

[mkoeppe]

rebased

[dimpase]

I guess this has to wait until Sage's CI is in order.

[mkoeppe]

@videlec @dimpase Ready for merging.

[dimpase]

Don't we want docs/Makefile and docs/source/ in MANIFEST.in ?

[mkoeppe]

Let's keep this PR focused on what is in its title please.

[dimpase]

I am asking a legitimate question - you added a Makefile to Manifest, I wonder whether there are more Makefiles etc that should be added.

[mkoeppe]

I am asking a legitimate question - you added a Makefile to Manifest, I wonder whether there are more Makefiles etc that should be added.

In this PR, no, because Sage does not do anything with cypari2 documentation.

Otherwise, also no, because when you install using pip from an sdist, the documentation is not to be built. This is something that developers would do directly in a git tree.

[dimpase]

OK, fine