optional=True should only be set for sdists

[smheidrich]

Reason: If wheel builds fail in CI (as has happened before, cf. #28), they'll be considered completed successfully, the CI run will succeed and the (empty) wheels uploaded when it is merged. We don't want that, we only want the sdist build on a user's machine to result in an optional extension.

One way to achieve this would be to have optional=False in setup.py and post-process the sdist and only set optional=True there. Similar to what I tried in #21 but more limited in scope.

[smheidrich]

Working on something for this: https://github.com/smheidrich/postprocess-sdist-make-rust-ext-opt