Pin astropy to 4.3.1, pin Sphinx to 2.0, pin jinja2 to 2.10

This MR rebases the head of celery5 against master and pins astropy to v4.3.1. The version of jinja2 currently required on master still has packaging issues in the CI pipeline, it spits out a lot of messages that say "ValueError: The 'gwcelery.jinja' package was not installed in a way that PackageLoader understands." Pinning jinja2 to 2.10, as is done on celery5, makes this issue go away. Pinning Sphinx to 2.0 also makes the Doc step pass successfully in the CI pipeline.

Assuming this MR is accepted, I think this should also become the head of celery5 unless there's a recent commit that doesn't belong on the celery5 branch. I'm not sure the best way to go about this is though, do I open a new MR or do we just rebase the celery5 branch against master?